Added assertRequest method

This method includes all parameters requested from Symfony's
BrowserKit\Client::request() method.  It works similarly to the assertGet
method, but will allow us to assert based on different HTTP methods.  It also
provides more flexibility when building requests.

The suggestion is to include both assertGet() and assertRequest() as
assertGet() provides a more lightweight way to assert GET requests.
Removing check for valid method

Because the HTTP standard does not require having one of the listed methods, we
do not need to check that the method exists in the enum, we can just pass it to
the request() method.
Merge pull request #5 from natebrunette/master

Added assertRequest method