Make color instance be a real instance

For now `(new Color) instanceof Color === false`, which makes unavailable Color instance detection.
The patch fixes this issue as well as adds detection of `Color` instance as an input argument.
All tests passes.

Please also update _color-convert_ dependency due to this PR: https://github.com/harthur/color-convert/pull/11.

I am going to use Color within [picky.js](https://github.com/dfcreative/picky) as a color model and this issue blocks normal way of making things.