Wallet transaction improvements

Summary:
Added method to get wallet user encrypted key
Added sendCoins which wraps several methods to create a tx, get the user key, decrypt, sign and send it
Added examples for sending coins (both the new and old way)
Added createWalletWithKeychains to make it simpler to create wallets
Added examples for creating wallets (both the new and old way)
Added tests for the above
Fixed some small bugs and renamed wallet.send to wallet.sendTransaction

Reviewers: ben, mike

Reviewed By: mike

Subscribers: ben

Differential Revision: http://phabricator.bitgo.com/D283