Add .libPaths() to R scripts for Windows

Hopefully a more proper attempt to fix http://hub.qgis.org/issues/11603 directly in processing by means of providing an option to define a path for user libraries on MS Windows. Not sure if this could be useful for other OSes too?

python/plugins/processing/algs/r/RAlgorithm.py

@@ -285,6 +285,8 @@ class RAlgorithm(GeoAlgorithm):
         commands.append('options("repos"="http://cran.at.r-project.org/")')
 
         # Try to install packages if needed
+        if isWindows():
+            commands.append('.libPaths(\"' + str(RUtils.RLibs()).replace('\\','/') + '\")')
         packages = RUtils.getRequiredPackages(self.script)
         packages.extend(['rgdal', 'raster'])
         for p in packages: