Build the message as part of the work item

Currently the message is built synchronously on the calling thread.
Generally this represents the majority of the call time as sending the
message is generally trivial despite being blocking I/O.

This adds the BuildMessage operation to the background task to minimize
the call time which I would believe to be more correct behavior for the
call.
Leave existing method as part of public API

Retain this for backwards compatibility
Fixed SendInBackground for the .Net 2.0 and 4.5 providers.
Fixed SendInBackground for the Xamarin iOS and Android providers.
Tweaked SendInBackground for WebApi support.
Handle WebApi constuctor errors better
Remove random tag data from RaygunWebApiExceptionLogger
Namespace cleanup
Extract HttpRequestMessage on the main thread before calling SendInBackground
Restore public members
Merge pull request #168 from MindscapeHQ/send-in-background-should-build-async

SendInBackground should perform the entire call asyncronously