Add Aviator.getCurrentRequest

Expose the current request via Aviator.getCurrentRequest (this calls
Navigator#getCurrentRequest and return the internal _request property
which is set during dispatch).

Internal API Changes (for better consistency):

* Add Navigator#getCurrentRequest
* Rename Navigator#getURI to Navigator#getCurrentURI
* Rename Navigator#getRequest to Navigator#createRequest
* Rename Navigator#getRouteForURI to Navigator#createRouteForURI

External API Changes:
* Add Aviator.getCurrentRequest
* Rename Aviator.getURI to Aviator.getCurrentURI

@barnabyc @flahertyb