Move ZlibDecompressor to libshared

* This will be used to implement compressed http streams
* Remove the custom BDataOutput class, and use BDataIO instead, for
easier integration with existing code.
ZlibDecompressor: also accept gzip format

zlib can decompress both gzip and zlib formatted compressed streams. The
compression algorithm is the same, but the headers are different. The
format is autodetected from the data.
DynamicBuffer: implement BDataIO

This makes it possible to use it with the ZlibDecompressor.
HttpRequest: support gzip and deflate compression.

* Use the ZlibDecompressor to decompress the data
* Advertise support in accept-encoding

This should make web browsing feel even faster on wesites that support
these compresion schemes. It also fixes some websites (www.ru,
rainloop.net, ...) that serve gzipped resources even to browser not
supporting it.