Update readme with new addin repository.

Add addin repository link for Xamarin Studio 4.1

Change Mono version tested on OS X to 2.10.12. Some menus
for Xamarin Studio do not appear when using Mono 2.10.11
with newer versions of Xamarin Studio. Version 4.0.1 on
OSX is OK but 4.0.9 and 4.1.6 had missing menus unless
Mono 2.10.12 was used.
Add application name to HTTP user agent.

Application name should be displayed in nuget.org/stats for NuGet
package downloads.
Use NuGet-Operation install HTTP header when adding packages from dialog.
Use NuGet-Operation update HTTP header when updating packages from dialog.
Use NuGet-Operation install HTTP header when manage packages button used.
Use NuGet-Operation update HTTP header when using Update All button.
Fix rootnamespace not being replaced in source code transformations (.pp)
Show that the Manage Packages dialog is looking for packages.

Add a searching message to the dialog list view when the
dialog is waiting for package information to be returned
from a NuGet feed.

Increase size of error and searching icons in Manage Packages
dialog to be a similar size to the NuGet package icon.
Fix bold highlight in readme.
Update default NuGet package url.
Update to NuGet 2.7
Update addin version to 0.6
Support restoring NuGet packages.

Add Restore NuGet Packages menu item that runs "nuget.exe restore"
for the solution.
Update release note.
Merge branch 'master' into xamarin-studio-4.1