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 
This seems like it will not play well with multiple SNe: on the first set of parameters, the observations are cut, and the cut remains in place for the next set of parameters (because the cut observations have been assigned back to `observations`). I'd probably just do

```
mask = (observations['time'] > tmin) & (observations['time'] < tmax)

flux = model.bandflux(observations['band'][mask],
                      observations['time'][mask],
                      zp=observations['zp'][mask])
```

Also wouldn't need the extra helper function.