Adding test for ColorInsValid
Moving constants for better readibility
Refactoring to reduce redundant code for byte encoding.
Using outPost instead of genesis.
Reducing even more redundant code for byte encoding.
Promoting MakeError public func.
Merge pull request #5 from vds/colorins_valid_test

Colorins valid test
Added additional method to interface.

Now checks if output has been spent first.

This closes #6