Add support for custom request matchers

This commit not only changes the default method of matching requests
(just match on method and URI instead of the entire request + headers)
but also allows the user to add custom matchers.

setup.py

@@ -39,7 +39,7 @@ setup(name='vcrpy',
       },
       install_requires=['PyYAML'],
       license='MIT',
-      tests_require=['pytest'],
+      tests_require=['pytest','mock'],
       cmdclass={'test': PyTest},
       classifiers=[
           'Development Status :: 3 - Alpha',