Extract contract metadata to its own crate (#69)

* Extract contract metadata to its own crate

* Add json serialization test

* Add json test for excluding optional fields

* Add doc tests and rename to contract-metadata

* Fmt

* Fix tests

* Fmt

* Builder API for contract metadata

* Tests for builder pattern

* Fmt

* Make metadata version camel case

* Builder optional fields

* Check whether fields have not already been set

* Check at least one author

* I really should check it compiles before pushing

* Oh yeah and cargo fmt fml

* Yeah