Fix NuGet packages showing as installed in solution.

NuGet packages that were still in the packages folder but
not installed in any projects are now not shown in the
Installed tab when managing NuGet packages for the solution.
Fix old NuGet packages not being removed on updating.

Updating to a new NuGet package or installing a new NuGet
package with the old one still installed would leave the
old unused NuGet package in the packages folder.
Update addin version to 0.8
Update readme

Add information on support for MonoDevelop and Xamarin Studio 4.2
Fix exception when NuGet feed results returned after dialog closed.

Remove property changed event handlers when the custom widgets
are destroyed. Fixes AggregateException thrown from task when it
tried to set the button text on the destroyed PagedResultsWidget.
Update testing doc.
Update to NuGet 2.7.2