updated r.project to 2.15.1 and added uninstall script.

R.Project/R.Project.nuspec

@@ -3,7 +3,7 @@
   <metadata>
     <id>R.Project</id>
     <title>The R Project for Statistical Computing</title>
-    <version>2.15.0</version>
+    <version>2.15.1</version>
     <authors>R Core Team</authors>
     <owners>Elliando Dias</owners>
     <summary>R is a system for statistical computation and graphics. It consists of a language plus a 

R.Project/tools/chocolateyInstall.ps1

-Install-ChocolateyPackage 'R.Project' 'exe' '/silent' 'http://cran.cnr.berkeley.edu/bin/windows/base/R-2.15.0-win.exe' -validExitCodes @(0)
+$url='http://cran.at.r-project.org/bin/windows/base/R-2.15.1-win.exe'
\ No newline at end of file
+Install-ChocolateyPackage 'R.Project' 'exe' '/silent' "$url" -validExitCodes @(0)

R.Project/tools/chocolateyUninstall.ps1

+$unpath="${Env:ProgramFiles}\R\R-2.15.1\unins000.exe"
+Uninstall-ChocolateyPackage 'R.Project' 'exe' '/silent' "$unpath" -validExitCodes @(0)