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.
Added additional method to interface.

Now checks if output has been spent first.

This closes #6
Corrections as a result of writing an integration test.

Also made btcd_test.go use table-driven tests and use constants like ifoc_test.
Also added a function to color.go for easily getting the color value at any outpoint.

This closes #9
Removed ColorId

This closes #11