This commit changes the whole core internal flow of requests.
Now, requests are actually physically made lazily when a response
is requested. This allows the entire request to be sent at once.
Otherwise, it would be impossible to compare whether requests have
already been recorded, since httplib.send() allows you to effectively
stream requests over HTTP.
This refactoring includes some PEP-8 compliance changes, as well as some more
complete testing to ensure that we're in fact serving everything out of
cassettes when we thing we are.
Incidentally, it also includes fixes for #3 and #4
