Gunzip support and autodetection based on URL.

Gunzip support via [zlib.js](https://github.com/imaya/zlib.js) gunzip library.

loader.loadFromURL now considers the option `compression` that controls whether
the response is interpreted as compressed and therefore decompressed
appropriately. `options.compression` can be set to:

 - `none` - no compression
 - `auto` - the default. Compression selected based on URL (e.g. gzip = .gz)
 - `gzip` - data is compressed using gzip