Re #6169. Make memory reporting field in xml report more compact.

I was hoping that the Jenkins Measurement Plots plugin would pick
it up and formatted it for that, but it seems that will need more
messing with the Jenkins config than I'm willing to do when the
main way of viewing this will be via the performances tests
infrastructure.
Re #6169. Use new API export of MemoryStats.

I was thinking of using psutil (http://code.google.com/p/psutil/)
but then realised that this would require it to be installed on
any machine that was going to run system tests. Also, it doesn't
seem to work on the latest Mac OS's.
However, I was pleased to see that its report of the available
memory on Linux & Windows matches exactly that given by Mantid.