[ADDED] First draft: sendmany RPC support

We can now run payouts via sendmany:

* No big logical changes, sendmany is called once for MP and once for AP
* Instead of paying out via sendtoaddress, store in array and use
  sendmany later
* Update all transactions with the RPC TXID returned by sendmany

Will fix #1238 once merged.
[CHANGE] Made startup messages debug level
[FIX] Apply TXFees before running sendmany
[UPDATE] Getbalance wrapper

We need to wrap our getbalance call due to issues in the coind and
payout systems in the RPC. Apparently it's having issues with multiple
accounts setup in a single wallet. Sendmany can not properly use other
accounts added to the wallet as the payout account.

Hence we wrap our getbalance call in the RPC and only return the default
account balance at all times. Since this is used for all payouts and for
payments from blocks, there should be no issue. Those pools running
multiple accounts in their wallet will have to move their coins into the
default wallet!
[ADDED] Sendmany config option

* Disable sendmany by default
* Allow people to enable the new feature via config
[ADDED] Log RPC TXID into logfile for payouts
[ADDED] Documentation URL
Merge pull request #1722 from MPOS/sendmany-support

[ADDED] First draft: sendmany RPC support