In Progress: Add $.ajax method

This should support:

- [x] Basic http requests
- [x] [Deferreds](http://api.jquery.com/jQuery.Deferred/)
- [x] As many of [jQuery's $.ajax settings](http://api.jquery.com/jQuery.ajax/) as possible (see below)
- [x] Global setup via $.ajaxSetup()

### Supported Settings ###

- [x] accepts
- [x] async (Only supported on iOS according to the [Titanium documentation](http://docs.appcelerator.com/titanium/latest/#!/api/Titanium.Network.HTTPClient-method-open))
- [x] beforeSend
- [x] cache
- [x] complete
- [x] contents
- [x] contentType
- [x] context
- [x] converters
- [x] data
- [x] dataFilter
- [x] dataType
- [x] error
- [x] headers
- [x] ifModified
- [x] mimeType
- [x] processData
- [x] statusCode
- [x] success
- [x] timeout
- [x] traditional
- [x] type (also `method`)
- [x] url
- [x] xhrFields

To consider:

- [x] password
- [x] username
- [ ] global - *This setting doesn't currently make sense without a DOM as a target for global events but may eventually be useful when combined with a global event handler such as Chaplin's `mediator`.*