Stealth address support

This pull request adds support for basic stealth addresses, i.e. stealth addresses with one scan key, one spend key and no prefix. This is the kind currently used in Dark Wallet. The code includes

* Calculating shared secrets
* Uncover pay pub key by sender and receiver
* Uncover pay private key by receiver
* Encoding pub keys to stealth addresses
* Create stealth transaction outputs

A unit test is also included to test the above. There is an implicit dependency on Pull Request #31, since the stealth transaction metadata outputs are of the form `{'script' : ...}`.