Implement tint and shade functions

[Synchro]

Replacement pull request for the messed up #947 (see that issue for discussion) which fixes #944, adding tint and shade functions, with tests.

[ricardobeat]

What I meant by inverting the amount (on #947) is this:

shade: function(color, amount) {
    return this.mix(color, this.rgb(0, 0, 0), 100 - amount);
}

That is, a tint(100%) of anything is white, and shade(100%) is black.

This is the approach taken by SASS too.

[lukeapage]

It looks symmetrical to me already... Your change would make it asymmetric.. or am I missing something?

[Synchro]

OK I see what you mean - the problem wasn't really inverting the amounts, it's that I'd got the order of parameters wrong in the call to mix - the amount parameter determines how much of the first colour is mixed with the second, not the other way around; that's now fixed. Also the test values chosen didn't make it obvious whether colors were getting lighter or darker, so I changed them to something more apparent, and added 100% cases.
I did squash these commits, but it still made me do a merge on push which was a little messy - I've also been reading that pushing rebases to public repos is frowned upon.

[lukeapage]

not sure what the merge is about.. I just rebase and then push it to a branch with force (since it overwrites the history on the target branch if you don't it warns you as you are deleting stuff) I think that is your problem - you must be rebasing fine locally then merging to your public repo branch and essentially undoing the rebasing.

What is frowned upon is re-writing history on a branch which other people are using for rebasing and merging - for instance If I decided to rebase cloudhead/less.js - it would screw over everyone who had merged the commit I squashed and git would get confused.

AFAIK there is no problem rebasing a public history if it is your own repo that no-one has branched.

[Synchro]

Yes, I need to force on push, thanks. Apart from that, is this pull request ok?

[lukeapage]

In the wake of 9 days with no criticism I pulled it.

[Synchro]

Thanks, Luke. You're doing a fantastic job BTW!