Adds a new NFluent.Tests.Extensions.40 project to fix the NCrunch covering issue (now both NFluent.35 and NFluent.40 are taken into account).

Upgrades the nunit version.

Removes useless dependencies within the NFluent.*.Extensions.* projects.
Unshares the AssemblyInfo.cs file between the NFluent.35 and NFluent.40 projects. Now, assemblies metadata indicates whether it's targeting .NET 35 or .NET 40