Added assert for form submissions

Works similarly to HaphazardTestCase::assertGet().  Requires an additional
string that defines what to search for to find the submit button.  It will
override the csrf provider to always return true when it checks if the token is
valid.
Merge pull request #3 from natebrunette/master

Added assert for form submissions
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
Added assertPost method

Added an assertPost method and converted both assertGet() and assertPost() to
forward to assertRequest().  Both assertGet() and assertPost() are simplified
versions of assertRequest(). Normalized variable names for consistency.

Also updated the documentation with new methods.
Merge pull request #6 from natebrunette/master

Added assertPost method
Updating packages to require at *least* 2.3 instead of exact
Adding in a factory method for creating the User Token

Broke the UsernamePasswordToken creation out into a separate overridable method
that just needs to return a TokenInterface object
Merge pull request #7 from MaxVandervelde/master

Adding in a factory method for creating the User Token