Flowchart example updated to use this feature.

- Essentially a graphical interface to dict; all items have text and value
- Assigns previously-selected text after list is cleared and repopulated
- Get, set current value

This merge converts all absolute imports into relative imports throughout pyqtgraph.
This allows pyqtgraph to be embedded as a sub-package of larger packages.

finished top-level files and graphicsItems

No more dynamic imports; pg uses static imports throughout.
Flowcharts and exporters use plugin systems

* Replaced dynamic imports with static
* Added NodeLibrary allowing multiple customized collections of Node types

  - CSV headers include data names, if available
  - Exporter correctly handles items with no data
  - pg.plot() avoids creating empty data item
  - removed call to reduce() from exporter; not available in python 3
  - Gave .name() methods to PlotDataItem, PlotCurveItem, and ScatterPlotItem

Added 'width' argument to PlotCurveItem.setClickable()

Added example of simple Designer usage.

Some Python3 related fixes.

* __init__.py now contains latest release version string
* installing from git checkout that does not correspond to a release commit
  will result in a more descriptive version string

setup.py now modifies __init__.py on build to include a more descriptive version string if .git is present.

New Profiler class from Antony Lee:
* Simpler API (with backward compatibility)
* Individual Profilers can be enabled by environmental variable PYQTGRAPHPROFILE

* Adjusted output formatting for clearer representation of nested profilers
* Message string formatting deferred until finish to reduce overhead

 * Loads only data that is currently visible
 * Downsamples to avoid plotting too many samples
 * Loads data in chunks to limit memory usage during downsampling

Added tools/setVersion script
setup.py now auto-generates version string based on pyqtgraph/__init__ and git info, if available

A new function profiling system is implemented.  Most importantly, this
allows one to profile various internal functions directly by setting the
`PYQTGRAPHPROFILE` environment variable to a comma separated list of
function and method names, e.g.

    PYQTGRAPHPROFILE=functions.makeARGB,ImageItem.render \
        python -mexamples

Specifically, items in `PYQTGRAPHPROFILE` must be of the form
`classname.methodname` or `dotted_module_name.functionname`, with the
initial "pyqtgraph." stripped from the dotted module name.

Moreover, the overhead of inactive profilers has been kept minimal: an
introspective check of the caller's name (only if `PYQTGRAPHPROFILE` is
set) and a trivial function (not method) call per profiler call.

The new profilers rely on `sys._getframe` to find the caller's name,
although the previous system (passing the caller's name explicitely)
could certainly have been kept instead.

Finally the API of profilers has been changed: register a
profiling point simply by calling the profiler, and profilers are
automatically flushed on garbage collection.  See the docstring of
`pyqtgraph.debug.Profiler` for more details.

- Speed improvements in functions.makeARGB
- ImageItem is faster by avoiding makeQImage(transpose=True)