Make the http lib swappable

This allows the user to inject their own http lib, and includes adapters for jquery and request http libraries.

jQuery is the default library, and it is included as a dependency. This will allow the user to configure qjax to work with the [request](https://github.com/mikeal/request) library, but the user will be responsible for including that library, by injecting it into the adapter, as seen in [this test](https://github.com/kahnjw/qjax/blob/make_http_lib_swappable/test/http-spec.js#L13).

This fixes: https://github.com/kahnjw/qjax/pull/12