Added kwarg to specify API Class you wish to use for communicate with Keen

This allows for other API classes to be specified other than KeenApi(). This allows users to implement things like API Classes that are specific to their framework. My use case is I need to use `tornado.httpclient.AsyncHTTPClient` rather than the default requests library for communicating with Keen.

It's a very simple change and I can't see anything breaks, all tests confirmed working.

```
➜  keen git:(master) nosetests
.....................
----------------------------------------------------------------------
Ran 21 tests in 47.299s

OK
```