Remove Microsoft.Build.dll dependency for Linux support

This adds a new assembly, Microsoft.Build.Evaluation.dll, and removes the reference to Microsoft.Build.dll, which is not supported yet in Linux/Mono.

new file:   Microsoft.Build.Evaluation/Microsoft.Build.Evaluation.csproj
	new file:   Microsoft.Build.Evaluation/Project.cs
	new file:   Microsoft.Build.Evaluation/ProjectItem.cs
	new file:   Microsoft.Build.Evaluation/Properties/AssemblyInfo.cs
	modified:   OmniSharp.sln
	modified:   OmniSharp/OmniSharp.csproj