Upgrade to BitcoinJ 0.12-SNAPSHOT

 - Update imports to reflect BitcoinJ's repackaging, including:

   git grep -l 'import com.google.bitcoin' | \
   xargs perl -p -i -e 's/import com.google.bitcoin/import org.bitcoinj/'

   git grep -l 'com.google.bitcoin' | \
   xargs perl -p -i -e 's/com.google.bitcoin/org.bitcoinj/'

 - Replace use of BitcoinJ's CoinFormat, which has now been renamed to
   MonetaryFormat, using:

   git grep -l 'CoinFormat' | \
   xargs perl -p -i -e 's/CoinFormat/MonetaryFormat/g'

 - Fix calls to BitcoinJ's Script#correctlySpends, whose signature has
   changed from 0.11 => 0.12.

See #98