Add #include to fix a compilation problem on gcc > 4.7.

This patch seemingly fixes a compilation error with Ubuntu 14.04 and
newer gcc versions.  See
https://github.com/chronokings/huntercoin/pull/50.
Merge pull request #117 from domob1812/fix-14.04

Fix compilation error in Ubuntu 14.04.
Start with UTXO database implementation.

Start with the implementation of a UTXO set database for tx
verification.  This is a squashed commit of the old utxo branch after
rebasing it onto the DatabaseSet changes.
Update the UTXO set in DisconnectInputs.

Also handle the UTXO set in DisconnectInputs.  Not yet really tested.