Add XML documentation

[StephenCleary]

Add XML docs in the code, generate the XML files during build, and include them in the NuGet packages.

[Mpdreamz]

We definitely need to write more xmldocs overall, its been a long standing issue and something we can hopefully block a week or two soon for.

We do not currently include documentation file for the netstandard1.3 artifacts in our nuget package, we'll remedy this ASAP.

net45 and net46 do already include sourcelink enabled XML files in the published nuget packages.