Add basic support for working with a widget's GdkVisual
added Ubuntu specific install directions

added some command line arguments for installing gotk3 on Ubuntu 14.04.
Add ProgressBar.Get/SetShowText.

We already had the .SetText method, but without being able to actually
enable showing text, it was rather useless.
Wrapper functions for GtkAdjustment
GetHAdjustment / GetVAdjustment for ScrolledWindow