Fix a typo in numbertheory.py, fix some docstrings.

Some improper formatting in docstrings was preventing
a few symbols from showing up in the documentation.
Fix IntegerExponent erroring on symbolic input.

IntegerExponent's pattern was changed to evaluate
only on [_Integer, _Integer] input.
Fix IntegerExponent erroring on symbolic input.

IntegerExponent's pattern was changed to evaluate
only on [_Integer, _Integer] input.
Add IntegerExponent test to verify symbolic behavior.
Fix minor typos and pare down some noisy function tests in the docs.
Merge pull request #108 from freshwater/typofix

Fix a typo in numbertheory.py, fix some docstrings.