feat(rating): add rating directive

[bekos]

Based on #337.

[pkozlowski-opensource]

LGTM after having a short look. +1 for merging it before 0.3.0 provided that is passes tests in all browsers.

[pkozlowski-opensource]

@bekos Had another quick look, added some minor points in the comments

But I still wonder if we shouldn't be using NgModelController here so people would be writing:
<rating ng-model="rate" max="10" readonly="isReadonly"></rating>

The advantage here would be that people could add custom things to a pipeline (data type conversion etc.). Beside the code would be smaller, no?

Normally I wouldn't care that much as we can always change it later but it would mean changing attribute name. Up to you.

Great work BTW, very clean PR and a nice addition to the set of existing directives!

[bekos]

I don't prefer ng-model because someone can also do something like this

<rating value="4" readonly="true"></rating>

but if you think otherwise I can change it. No strong opinion here.

[pkozlowski-opensource]

@bekos I see your point about specifying a non-assignable expression. I don't have strong feelings either way, will leave the final decision in your hands. Unless others want to weight here.

[ajoslin]

👍 Looks good. :-)

@bekos I think you are right, that readonly is a valid use case.

[bekos]

Comments are fixed.
I will keep thinks as they are, since and @ajoslin agrees ( I'm not used to this :-) )

@pkozlowski-opensource Thank you very much for the comments :-)

[pkozlowski-opensource]

@bekos squashed commits and landed it as 6b5e636.

Great work!