Add XML decoder

- based on @mcaserta's work at https://github.com/netty/netty/pull/1121
- not ready for a merge yet
Fixed the XML decoder
CDATA support
Fix inspector warnings
Fix buffer leaks in XmlFrameDecoderTest
Fix another buffer leak in XmlFrameDecoderTest
Change the return type of EmbeddedChannel.read*() from Object to an ad-hoc type parameter

.. so that there's no need to explicitly down-cast.

Fixes #2067