circular-dependency refactorings

This should resolve @mbakkar circular dependency issues #1622 and allow to use codecs while implementing socket proxy support

merge netty-buffer with netty-common cause buffer is always used with
common modules
move common Channel classes and interfaces under netty-common
modules(codecs should not rely on transport module)
move EmbeddedChannel under netty common (it is always used for testing
implementations)
removed obsolete dependencies to netty-common because it is in other
modules dependencies
rewrote SpdyFrameDecoderTest to use EmbeddedChannel instead of NIO,
remove codec-http dependency to transport-module

@normanmaurer  please review