Fix Issue 47 - Crash while closing application

Wait for read loop to exit in destructor.  Needs to be tested.
increase default name mining difficulty to several minutes
added new messages for email storage
update proof-of-work validation
fix receive messages on load
Merge remote-tracking branch 'upstream/master'
tongbu upstream
Merge branch 'master' of github.com:vipjeffreylee/BitShares