* Introduce `ContractAddress` newtype instead of scheme enum

Not all variants of scheme used all the parameters of the previous
`contract_address` function, yet they still needed to be passed.

* Pass code_hash instead of hashing code inside the function

This allows us to not return a tuple but just the address.

* Fix README to reflect actual usage

* Bump version to 0.2.0 to reflect breaking API change

* Add build setup to .travis.yml to ensure example in README is up-to-date