Add test: test_session_and_connection_close

This is a test for issue GH-48.
del self.sock in VCRConnectionMixin.request

instead of in connection class constructors.

Fixes GH-48.
Fix Cryptic 'write-protected' Message

Closes #46
Fix API by adding 'responses_of' method

`responses_of` replaces `response_of`, since each request can have
several matching responses now.

This breaks backwards compatibility if you are using the
response_of method, so a version bump will be required.
formatting fixes
Version bump to 0.5.0