Adding Doppler, LOG and identity WCSes

Added the Doppler, Log, Identity WCS with documentation. Created a new PR for clear differentiation.
@nhmc @wkerzendorf How is this looking? 

In the end every WCS should be nothing else but a function that takes input (pixel_indices) and gives out put. 
So your Combination WCSes should be as you already have done on some level a list that carries multiple models and then applies them successively. https://github.com/astropy/astropy/blob/master/astropy/modeling/core.py#L950 defines the `SerialCompositeModel` and while I think that one is not quite there yet to inherit from I think we want to make this the general Structure. A Model that holds a list of Models and calls them successively. So in your `IRAFCompositeModel` this would be for each WCS that you find in Multispec make an individual `CompositeModel` with the "intrinsic" WCS as well as the doppler shift and then make a `CompositeModel` out of the different `CompositeModel`s. Here's a Pseudo example:

```
[[WCS1 -> Doppler1 -> Weight1] -> [WCS2 -> Doppler2 -> Weight2] -> ... -> [WCSN, DopplerN, WeightN]]
```
Where the arrows show how a number trickles through it and the square brackets indicate a CombinationModel. This Model will allow you to represent all IRAF WCSes, but have a much more general structure and thus will be much more powerful. 

Does this make it clearer?