Fix bugs in encoder.js

These changes made the encoder work for me. Tested using the example in the README (with a 32-bit float PCM file instead of stdin) and it worked marvelously.

The big issue seemed to be the attempts to use "output" (which, in the scopes involved, I understand to be the "encoding" parameter (a string) provided by the Transform API) as a function for writing the OGG data. The docs [here](http://nodejs.org/api/stream.html#stream_transform_transform_chunk_encoding_callback) showed that transform.push(buffer) is the appropriate way of doing that.