Add support for calling httplib.send().

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.
Add support for configurable record modes

This support will let you select one of four different behaviors
for VCR's cassettes.  Closes #23
Add support for custom request matchers

This commit not only changes the default method of matching requests
(just match on method and URI instead of the entire request + headers)
but also allows the user to add custom matchers.
bump development status to Beta