Add before_record_response callback to use_cassette; Various bugfixes

• Move context management of use_cassette into CassetteContextDecorator so that a separate cassette is created for each invocation of a use_cassette decorated function.
• VCR now passes a callable to CassetteContextDecorator (instead of passing the arguments for cassette creation) so that changes to the VCRs default configuration can propagate to functions that have already been decorated.
• Add mock and contextlib2 as dependencies.
• Do all patching through mock.patch. This made it so that CassetteContextDecorators actually stack properly.
• Remove enter and exit from Cassette class.
• Add before_record_response hook to Cassette and VCR classes. This is mainly for the purpose of scrubbing responses.
• Version bump (1.0.3 -> 1.1.0).
• Remove contextdecorator dependency.