Support sending to P2SH addresses

Hi,

This pull request will enable My Wallet to send to P2SH (3XXX) addresses.

The motivation behind this is to allow interoperability between Bitcoin wallets in handling multisignature transactions, improving security and enabling new applications (such as arbitration).

Currently, Electrum, bitocoind and Coinbase support sending to P2SH.  BitcoinJ wallets are slated to support P2SH in the next release cycle (including Android Wallet and Multibit).

Note that I didn't update bitcoinjs.js and bitcoinjs-min.js since I couldn't find the minify.sh script.  I also see some differences between the jsbn that is in bitcoinjs.js and the source files.

Let me know if any adjustments are needed to the pull request.