Added install script; use iterative outlier rejection to calculate scale

*  Install script will need to be added to, but this works for development.
* ```Noise``` now accepts a file name and instantiates a default ```SpectralCube``` object 
*  ```sigma_rob``` is now used to calculate the scale.
*  ```self.snr``` now a property
*  Tests values have been updated 

This address issues #21 and #22.
Should this be `function` instead of `mad`?
Also, some navel-gazing here, this works for positive outliers given the `<=` sign.  Should this instead trim to be `thresh*noise` around a central value estimate?  So something like
```
ind = (np.abs(data - medval) <= thresh*noise).nonzero()
```