BUG: Determine cause of floating point exception with accelerated __mod__
For some reason, using numexpr with `%` and zeros causes floating point exceptions. I.e., in [this Travis CI test](https://travis-ci.org/jtratner/pandas/jobs/7917801) you can see the mysterious floating point exception occur:

``` bash
#snippet from nosetests
........................................................S.................................................................
ci/script.sh: line 13:  4513 Floating point exceptionnosetests --exe -w /tmp -A "$NOSE_ARGS" pandas
```

The exception occurs when `__mod__` is evaluated with str_rep of `%` (meaning `a_values % b_values` is passed to `numexpr.evaluate`), and it occurs in `test_expressions.py` under `test_mixed_floats_arithmetic` and `test_integer_with_zeros` in (for example) jtratner/pandas@e938da2c5e28171488392fa2605e4e18a47c65eb . Omitting `str_rep` from `__mod__` fixes the issues, clearly showing it's occurring with numexpr.  Going to investigate more in the future.
