Added functionality to simulations.realize_lcs to simulate light curve
data at a subset of observation points among those in an observation
table, where the selection is based on a time window around peak. The
time window can be defined by the user, and is convenient to define such that
all SN are within a particular window of phase for all SN, irrespective
of redshift.
- The feature is used by default with a phase window of (-30, 60) days
- The value can be changed by setting the argument phaseWin=[tmin, tmax]
  of the simulations.realize_lcs function.
- The feature can be turned off by setting the argument phaseWin=None
pep8 fixes to simulation.py
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
Merge branch 'master' of github.com:sncosmo/sncosmo into realize_lcs_tw
add test for nest_lc
- Fixed problem of cutting on later observations.
- Current example code expanded to check for this at: https://gist.github.com/91e1d2a083fd1822b05f
- Would have hoped to be able to call

  >>> sncosmo.plot_lc(data, model=model.set(**p)),

  but that does not work, instead I need

  >>> model.set(**p)
  >>> sncosmo.plot_lc(data, model=model)
Merge pull request #53 from rbiswas4/realize_lcs_tw

Closes #52
minor clean-ups from PR #53