Drop the decode lock on stream handling.

This lock was actually detremental to the speed since we piped streams directly to the Bitmap decoder rather than through an intermediary byte array. Thus we potentially blocked all bitmap decoding while waiting on a single stream.

Only lock decoding from Bitmap to Bitmap since that is where the real potential to OOM exists.

Closes #49.
Merge pull request #57 from square/jw/pop-lock-and-drop-it

Drop the decode lock on stream handling.