Add XML documentation to public members

[martincostello]

Public members of the assemblies we ship via NuGet should have appropriate /// documentation so that consumer of the libraries get nice Intellisense documentation, making more "traditional" documentation of methods, properties etc. less important.

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[martincostello]

This is still being worked on.

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. Thank you for your contributions.