adding xruns_get command

So you likely don't want to merge this as is but I wanted to draw your attention to it anyway. This gives you an ```xruns_get``` command which responds: 

    resp 0 <number of xruns> <max delay usecs>

It resets the values when invoked. I am planning on using mod-host to evaluate the performance of a number of SoCs + Codec hardware combinations which is what I need this command for. I am not sure how useful it is to you guys.

I pulled in jack2 as a submodule so I can use JackAtomic.h though so if you do want to merge and are not happy with that then I am open to suggestions. Well, I am open to any comments or suggestions anyway actually.