POS mining performance. Kernel stake modifiers.

Make POS mining even less burdensome by storing
off the kernel stake modifiers when they are first
calculated and then re-using them.
Merge remote-tracking branch 'origin/pausePOSmining'
Added QT build section to README_ubuntu.txt

A new section is added to the bottom to show
how to build the QT client version of YACoin
on Ubuntu 12.04 Desktop.
Merge branch 'ubuntuReadmeUpdate'
A fix for getbalance "account"

The RPC command getbalance "account" returns random
numbers. This fixes that.
Merge branch 'getBalanceFix'
Merge branch 'master' into MSVCport