Add support for Promises (part 2)

[rozele]

In addition to supporting a promise data type in the ReactMethods, we should also add support for invoking asynchronous methods and chaining the result to a promise. This needs to be managed very carefully, as the native modules are run using the "AnyBackgroundThread" spec; the concern is that if the asynchronous call releases control too early, it may allow the next item in the native queue to execute.

[rozele]

Also, ensure that running asynchronous operations will not compromise the LimitedConcurrencyTaskScheduler.

[rozele]

In general, this is not a good idea. Primarily because the user should be very conscious of what sort of asynchronous or blocking activities they are performing on one of the "main" module threads. In general, the guidelines are to move any asynchronous work off thread. It's fine to use async void, so long as the developer knows what they are doing (e.g., ensure use of ConfigureAwait(false) where necessary).