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
Removing Pypy tests until travis installs version 2.2.1
Add Decorator Support
Store Headers as a List

This is a backwards-incompatible change that will store headers
as a list rather than a dictionary.  The reason being that you can
have multiple values for a single header, and concatenating them
together with commas can create an unparseable string (sometimes
the header values can have commas in them)
Nicer error when cassette doesn't contain request

Raise UnhandledHTTPRequestError.
Show name of cassette and request.
Nicer error for can't overwrite existing cassette

Raise CannotOverwriteExistingCassetteException rather than Exception.
Include cassette filename and record mode in error message.
Let's have the new exceptions subclass basic exception types
PEP8 Fixes
Load Old Cassettes

Add backwards-compatible cassette loading code that can load
the old style header dicts from response cassettes.
Update README.md
tox.ini: Add {posargs} for passing args to py.test

This allows you to do stuff like:

   tox -e py26requests,py27requests,pypyrequests -- tests/integration/test_requests.py
patch: Save requests...HTTPSConnection

so that we unpatch back to the correct class in reset().

Closes #59
Merge pull request #63 from msabramo/issue_59_save_requests_HTTPSConnection

patch: Save requests...HTTPSConnection
Merge pull request #61 from msabramo/tox_posargs

tox.ini: Add {posargs} for passing args to py.test
Add note to README about tox usage
README.md: minor formatting, add links
Merge pull request #65 from msabramo/patch-1

README.md: minor formatting, add links