Add GaussianAbsorption1D

Add `GaussianAbsorption1D` model for Gaussian absorption line modeling, as discussed on astropy-dev mailing list.

Here is an example of using this new model to fit absorption line (based on existing `astropy.modeling` 1D fitting example in the documentation):
![figure_1](https://f.cloud.github.com/assets/2090236/2463434/f03e02e0-af8b-11e3-901a-fbe32c5fd51e.png)

@astrofrog, "I don't feel strongly about whether your new model should be included."

@embray, "In fact, it would be a good test for composite models -- implement GaussianAbsorption1D as a composite model and see how the results compare to the hard-coded GaussianAbsorption1D."

@wkerzendorf, "This looks great for an idea for composite models!! +1"