Merge pull request #22 from stampsy/xamarin-pr-avoid-starting-tasks-where-possible

Use TaskCompletionSource instead of Task.Factory.StartNew where simple result is needed
Update iOS csproj targets
Don't block threadpool threads waiting for request

Resolves #23
Merge pull request #24 from stampsy/xamarin-pr-preserve-synchronization-context

Preserve synchronization context because OnRetrievedAccountProperties uses it
Merge pull request #29 from stampsy/xamarin-pr-expose-requestaccesstokenasync

Expose RequestAccessTokenAsync down the hierarchy
Merge pull request #30 from stampsy/xamarin-pr-make-oauth1-header-configurable

Make OAuth 1 header generation overridable
Merge pull request #25 from stampsy/xamarin-pr-dont-leave-task-exceptions-unobserved

Propagate GetAccessTokenAsync exceptions to the calling code
Make request body configurable for service implementors. This allows for Facebook batch requests.