Begin constructing table of values answer type.
Adding answers and change how to set headers.
Check answers for table set of values.
Remember entries in rows/columns that are hidden.

This way the number of columns and rows can be safely changed
without loss of data.
Add a table of values answer type.

Summary:
For now, the table of values answer type will take a set of
tuples (pairs, or single values) that must all be provided, but can
be given in any order.

Test Plan:
Add a table widget, or set the answer type to table. Change the
columns and rows to be higher than the default. Enter in values in random
cells, also trying mathjax in the column headers. Then reduce and
subsequently re-expand the table size to see if the entries are still
present.

Reviewers: alpert, alex

CC: eater

Differential Revision: http://phabricator.khanacademy.org/D2657
Score table of values using khan-exercises answer types.

Test Plan: Check to make sure that correct answers score correctly,
invalid ones score invalid, and wrong ones score wrong.

Auditors: alpert