Added nuget dependencies for EntityFramework

The two Npgsql.EntityFramework nugets were missing
dependencies on EntityFramework itself, added those.
Added nuget EFProvider installation

Added nuget Powershell installation script for
Npgsql.EntityFramework, which adds the Npgsql
EF provider into the project's App.config, as
recommended here:
http://entityframework.codeplex.com/wikipage?title=Rebuilding%20EF%20providers%20for%20EF6

Note that this wasn't done for legacy EF