Revert "Add global toggle to use_cassette."

This reverts commit 366e2b75bbba851325e63499c9770ebcf8f3faf7.

Conflicts:
	tests/unit/test_cassettes.py
Fix failure in test_local_host resulting from attempting to add tuple to list.
use_cassette -> CassetteContextDecorator
Change use_cassette to pass a function to CassetteContextDecorator so that changes to the default settings on the vcr properly propogate.
Failing tests for nested context decoration.
Use mock for patching http connection objects.
Python version agnostic way of getting the next item in the generator.
Check for old style class when building subclass.
hasattr check for requests 2.0 use cassette added type for httplib2 dictionary patch.
Refactor build_patchers into class. Fix issue with patching non existent attribute with hasattr.
Use contextdecorator from contextlib2. add logging for entering context.
Add test that fails because of the fact that a new class is used for each cassette context instead of replacing the cassette on the existing mock connection.
Fixed issue in test_nested_context_managers_with_session_created_before_first_nesting. by using a single class and patching cassette on that class. Not a great solution :\
Add failing test for session still being attached to cassette after context is gone.
Use {[testenv]deps}, instead of repeating testing requirements. Write another failing test for #109
Revert "Fixed issue in test_nested_context_managers_with_session_created_before_first_nesting. by using a single class and patching cassette on that class. Not a great solution :\"

This reverts commit 2bf23b2cdf8542f5bf137e386f3bc278bbc2cc83.
Fix some of the issues from #109
Mark bad test xfail.
Remove ConnectionRemover class that tried to get rid of vcr connections in ConnectionPools.
Make changes from b1cdd50e9b633060a801e9f8669de375fbe946c9 compatible with requests1.x; Update Readme.md with description of `before_record_response`