Added (beta) support for sending requests with context

Added the ability to pass in (one) state in the context field, as described in the [state](https://wit.ai/docs/intro#toc_40) docs. While this method can still be used to toggle capturing voice intent:

````Objective-C
[[Wit sharedInstance] toggleCaptureVoiceIntent:NULL];
````

This method can also be used, to add the optional state in:

````Objective-C
[[Wit sharedInstance] toggleCaptureVoiceIntent:NULL withContext:@"state_name"];
````