channels: Added the hardware impairments modeling tools from Matt Ettus' GRCon11 talk.

Python blocks part of the gr.channel module.

Examples are in gr-channels/examples (demo_gmsk, demo_qam, demo_ofdm, and others).
qtgui: Added histogram sink.
channels: using new qtgui histogram sink in quantization demo.
docs: added more documentation to the channel models page explaining the diffenerent models available.

Includes discussion of the new HW impairments model that is Pytho-only.
qtgui: improved handling of scale in histogram plots.

Mouse scroll zooms in/out horizonatally. Added auto-scale feature, can set x-min and max in constructor.

Mouse scroll in constellation plot now does both axes at the same time.
channels: updated quantization demo for new histogram constructor.