Add Network Hashrate to getmininginfo

2 functions were added to rpcmining.cpp: GetNetworkHashPS and getnetworkhashps.  In addition, the network hashrate was added to the array returned from "getmininginfo" at index "networkhashps".  These changes should not have any effect on the core functionality of the wallet or network.  The wallet with these changes is currently running on becausePool.com with no issue.