add depositAddress()

Hi. I added a function to get a Bitcoin deposit address (requires the Deposit access right), called 'depositAddress', based on:

* https://bitbucket.org/nitrous/mtgox-api/overview#markdown-header-money#markdown-header-moneybitcoinget_address
* https://github.com/pathaksa/MtGox-PHP-API-V2/blob/f523990cc1a36b95f7c5d8925dff58e9ab71f8b8/MtGoxClient.php#L273

Thanks for sharing this library, very useful and clean.