WIP BIP39

This PR implements mnemonic seeds as described in https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

Some screenshots of how it would work in Copay:

![image](https://cloud.githubusercontent.com/assets/2073468/5543273/cd02079c-8ad0-11e4-93f8-c4750f8fdd69.png)


![image](https://cloud.githubusercontent.com/assets/2073468/5543279/dfa000b6-8ad0-11e4-8db2-e40b84cb0a74.png)


![image](https://cloud.githubusercontent.com/assets/2073468/5543294/048d8c2c-8ad1-11e4-8fd6-0aa571ff2d63.png)

![image](https://cloud.githubusercontent.com/assets/2073468/5543314/24f53032-8ad1-11e4-9a60-e7a220cb9108.png)


![image](https://cloud.githubusercontent.com/assets/2073468/5543315/3993d476-8ad1-11e4-81f0-c442730b748a.png)


![image](https://cloud.githubusercontent.com/assets/2073468/5543327/5f7d0748-8ad1-11e4-9f1e-9b03df5248fa.png)