Very basic sign & verify message implementation

Needs some (a lot of) work on UI/UX, as probably you have your own idea that suits better for the whole wallet. There're some details yet to be done, like maximum length of message or the way the addresses are displayed to sign a message (copied it from the receive screen, but you might want to add a label to frequent addresses for example, to not mix them up)

I also thought about combining both actions under the same menu option, as it's now it occupy a lot of space in the menu and they're not so frequent actions.


Sign action cancelled
![trezor_sign_cancel](https://cloud.githubusercontent.com/assets/475423/4224987/34d6c1bc-392a-11e4-8118-939b0bfbe5b3.png)

Sign succeeded
![trezor_sign_success](https://cloud.githubusercontent.com/assets/475423/4224986/34d2f21c-392a-11e4-8ac6-6e34efc00438.png)

Verify message failed
![trezor_verify_error](https://cloud.githubusercontent.com/assets/475423/4224988/34d8410e-392a-11e4-9fe6-53e8e5d37e75.png)

Verify message succeeded
![trezor_verify_success](https://cloud.githubusercontent.com/assets/475423/4224989/34db6b7c-392a-11e4-9bfe-7268e7f6d23e.png)


Any advice is welcome, while this works, I do not feel it's ready to be release/merged yet.