Remove compressed argument from PrivateKey and PublicKey constructors

Rationale:

* Bitcore v0.8 goal is to simplify interfaces and reduce the knowledge required by the user-developer.
* The `compressed` format is the standard way to represent a private/public key.
* If a new key is generated it should be using the standard format.
* Allowing the user-developer to create new random keys using the uncompressed format adds unnecessary complexity.
* Creating a key from an `uncompressed` format must work though.
toJSON should return an object