Implemented: added ability to assert the implementation of a specific interface
Documented: added new assertion to the list of assertions
Fixed: corrected naming of the arguments to be a better fit to the librarys pattern.
Merge pull request #15 from lapistano/implementsInterfaceAssertion

Implements interface assertion
Implemented assertion to verify urls using the filter_var() function of PHP
Refactored: beberlei pointed me to some strange behaviors of filter_var(). So I adapted SF2 code for a more secure assertion.
Merge pull request #19 from lapistano/moreAssertions

Implemented assertion to verify urls using the filter_var() function of PHP