Add POST tests and change behavior for sending empty POST requests.

Empty POST requests now return HTTP 205 codes, since they indicate that the client should reflect the fact that the playlist has been reset.
Fix a few issues with the POST tests.
Add and test better error handling for malformed POST requests.

An integration test for POST requests has been written, and some server code has been fixed to make the test pass. Muxamp can now appropriately respond to input where the save request contains invalid tracks.
Add an identity map to the stream module to mitigate the effects of any API quotas.

I noticed while testing a large playlist that YouTube may occassionally return the too_many_recent_calls error (https://developers.google.com/youtube/2.0/developers_guide_protocol#Caching_data_to_avoid_quota_issues). As per the documentation's suggesstion, tracks will be cached for 30 minutes after being fetched. While an hour is suggested, 30 minutes will use a smaller memory footprint.