Add support for \overline

Summary:
Follow the instructions in the TeX book for drawing \overlines. This uses the
same code as fractions to produce the bars. Also added the ability to cramp
styles (i.e. T -> T' and T' -> T').

Test Plan:
 - Look at `\overline{x}`, `\overline{\dfrac{x}{y}+z}`, and
   `\blue{\overline{x}}` to make sure they look good.
 - Make sure the tests work
 - Make sure the huxley tests look good (Not here yet :( )

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D11604