Feature: paper-progress-circular component

[peec]

• Converted from ember-material-design
• Added tests
• Uses the new color mixin just as paper-progress-linear.

[miguelcobain]

Perfect!
Just two things:

1. I'm trying to avoid any md- in the public API, just like I did with paper-linear-progress. Can we do the same here?
2. Can you update the CHANGELOG.md

[peec]

@miguelcobain Ah, didn't notice the md attributes, updated PR with removed md- syntax and changelog.

[miguelcobain]

Tests are failing. #137

I merged the changes anyway, because I believe nothing was changed on ember-paper to cause those fails.

Also added some logic that allows progress-linear and circular to infer the mode they should work on based on the passed in values. I believe this makes the API a bit friendlier.

Excellent work. Thanks a lot.