Add a11y to the calculator

In this PR are added a11y things described in [BLD-165: Calculator hints tooltip](https://edx-wiki.atlassian.net/browse/BLD-165) and [BLD-164: Calculator button role and states](https://edx-wiki.atlassian.net/browse/BLD-164).

To see the calculator, you have to set the advanced setting "show_calculator" to true.

What should be done described in the page 23: https://docs.google.com/a/edx.org/file/d/0Bz_PqumfHVunMEFpNnFzQm0yY0E/edit

@jmclaus @frrrances please review.