Content encoding

This PR adds the ability to read message bodies that have been compressed with the `gzip`, `compress`, and `deflate` content encodings (see RFC 2616 sections 3.5, 14.3, and 14.11). This PR includes code from a 3rd party library, [lzwj](https://code.google.com/p/lzwj/), for decompressing LZW (`compress`) data. The 3rd party library has been acknowledged in the `NOTICE` file.