more unit test coverage

[incanus]

Lower priority for now since we've determined what we need to from writing the initial tests, but Xcode 7 shows some lack of coverage:

We should be exercising things like color-to-hex and vice-versa.

[1ec5]

Test coverage has improved significantly since this issue was written. As of #54 (comment), we’re at 76.97%. As far as I can tell, most of the improvement has come from streamlining the library rather than from adding more tests.