Plugin system

Hi guys,

I've merged the sma-wideband fork with ska-sa and it went smoothly. Here's my pull request which should merge easily for you.

This adds the plugin system I've written as well as some minor changes I merged in from Dave at some point. I have tested that the code builds successfully on my x86_64 machine but have not tried this on a ROACH2. I ran it locally on my machine and tested a simple plugin but like I said I haven't tried it on a ROACH2.

Once we're merged I'll add an example on the Wiki with more information on how to use the plugins.

Thanks,
Rurik