Support for new events APIs

[AndrewHartAR]

The new events APIs require a JSON body to be attached to each request, for example with Direct Messages:

https://dev.twitter.com/rest/reference/post/direct_messages/events/new

There doesn't appear to be a working way to do this with STTwitter currently. Passing in a JSON body as a parameter dictionary causes an error on encodedParametersDictionaries when it attempts to string encode the first value, which itself is a dictionary.

[mironal]

related issue #282