Fix #98

[jchodera]

This fixes the warning

/cbio/jclab/home/hansons/opt/anaconda/lib/python2.7/site-packages/assaytools-0.2.0-py2.7.egg/assaytools/pymcmodels.py:239: RuntimeWarning: invalid value encountered in divide
  scaling = (1 - np.exp(-alpha)) / alpha     

that would not actually cause problems because the nans were replaced by numerical values in the next line. Instead, we split up the calculations into two branches so as to not trigger this warning in the first place.

[sonyahanson]

Looks good. This is good for merging?

[jchodera]

Yep!