Add handling for answer strings that convert to float inf and nan

@mulby @brianhw 

This fixes a very interesting bug we ran into.  The symptom was the following SQL query error on insert
```
1054 (42S22): Unknown column 'inf' in 'field list'
```
which turned out to be caused by the following data:
```
(u'Education/EDUC115-S/Spring2014', u'i4x://Education/EDUC115-S/problem/1008ec10508c4b2c99930e47b1f92532', u'i4x-Education-EDUC115-S-problem-1008ec10508c4b2c99930e47b1f92532_2_1', u'1', 1, None, u'infinity', inf, None, u'Quiz - Fractions: Big Ideas', None)
```
What's going on is that `float(u"infinity")` evaluates to `inf`, which breaks the mysql connector prepared statement.  So this converts the output of any `inf` or `nan` to `None`