Performance boost to POS mining

When there are 250+ inputs eligible
for POS, the yacoin-qt client gets bogged
down by the automatic POS miner. This
performance fix speeds things up by
creating a hash table for the POS miner
thread to use to avoid calculating block
header hashes for every input over and over.
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.
Update checkpoints.
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 pull request #36 from grokouser/ubuntuReadmeUpdate

Added QT build section to README_ubuntu.txt
A fix for getbalance "account"

The RPC command getbalance "account" returns random
numbers. This fixes that.
Merge pull request #37 from grokouser/getBalanceFix

A fix for getbalance "account"
Merge pull request #34 from grokouser/POSperformance

Performance boost to POS mining
Merge pull request #39 from yacoin/master

Latest changes to testing.