Added GreenAddress wallet

This change adds GreenAddress to the wallet list.

GreenAddress has been in operation in various incarnations since [2013-04-16](http://blockr.io/tx/info/3af0d7e47a9e29b2866883b175f7d529fde8dfd448f7bdc1912d2dea7c0fe5fb). GreenAddress operates similarly to BitGo in that the user holds the keys, but GreenAddress is different from Electrum, BitGo and bc.i in that wallets are BIP0032 and BIP0039 P2SH/2-of-2 multisig. Also we don’t use passphrase-encrypted private keys, but Electrum-like random mnemonic for better security. You can read about how it works in our [FAQ](https://greenaddress.it/en/faq). We've been helping the Bitcoin community with some [Electrum server patches](https://github.com/spesmilo/electrum-server/commits?author=jkozera), [Gentle](https://github.com/greenaddress/gentle), a tool to help deal with transactions with expired nLockTime as well as our [own open source clients](https://github.com/greenaddress/).

Let us know if you have any questions.