In Progress: Add $.ajax method

This should support:

- [ ] Basic http requests
- [ ] [Deferreds](http://api.jquery.com/jQuery.Deferred/)
- [ ] As many of [jQuery's $.ajax settings](http://api.jquery.com/jQuery.ajax/) as possible (see below)
- [ ] Global setup via $.ajaxSetup()

### Supported Settings ###

- [ ] accepts
- [ ] async (Only supported on iOS according to the [Titanium documentation](http://docs.appcelerator.com/titanium/latest/#!/api/Titanium.Network.HTTPClient-method-open))
- [ ] beforeSend
- [ ] cache
- [ ] complete
- [ ] contents
- [ ] contentType
- [ ] context
- [ ] converters
- [ ] data
- [ ] dataFilter
- [ ] dataType
- [ ] error
- [ ] global
- [ ] headers
- [ ] ifModified
- [ ] processData
- [ ] statusCode
- [ ] success
- [ ] timeout
- [ ] type (also `method`)
- [ ] url
- [ ] xhrFields

To consider:

- [ ] password
- [ ] username