Added "deb" setup command

- fixed PlotCurveItem generating exceptions when data has length=0
- fixed ImageView.setImage only working once

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