Changed simulation.py to what I need, need to test, and chnage docs
Added changes to reflect discussion on PR and gitter.
- Now, set the observation window to model time range or keep fluxes for
  all observations.
- default: keep fluxes for all observations
- controlled by keyword argument: trim_observations
- keeping observations to modelrange uses model.mintime() and
  model.maxtime(), so can be extended beyond SALT2 in a consistent manner.
- improved testing to account for two SNe to check behavior on list.
  gist is at http://nbviewer.ipython.org/gist/anonymous/ae927673313dfea5bbe8
remove now redundant helper function _cut_obstable
pep8 fixes:
- test snapshot:
  http://nbviewer.ipython.org/gist/anonymous/bc1630ca064f2398ca7f