Realize lcs tw

Attempt to address https://github.com/sncosmo/sncosmo/issues/52. 

- tested on observation set in the form of `~astropy.table.Table` http://nbviewer.ipython.org/gist/anonymous/7c89197fb5e0df96b25d
- not sure about how this will work on the other allowed argument form of observations in realize_lcs(). I think this should work whenever observations['time'] > val returns a numpy boolean array.
- Maybe not doing the selection/conversion efficiently enough, speed has been increased in my example (10 years) by about 30 times for a single SN. Have not profiled to figure out where time is being spent