Adding DVB-T dongle and other components

Hi Paul, 

I've updated the DVB-T component for Iris. I've tested it with the dongle on Ubuntu 12.04. I've also added some spectrum analyzer components created a long time ago by Wei Liu. I don't know how useful these are as you are probably adding in some graphical components but at least they can be used for now. 

I've added in an example showing how the components can be used with a DVB-T dongle to create a simple spectrum analyser.

In terms of dependencies, rtl-sdr needs to be installed from git://git.osmocom.org/rtl-sdr.git to use the rtl dongle. This is pretty straight forward. The plotting tool relies on gnuplot, which can be easily installed using apt-get. 

Let me know if you have any questions!
Jonathan