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? 

I don't think that is right. When you put z=1 then velocity should be c, right? I think it is easier to make this conversion in `from_doppler_factor` and then call that in `from_redshift` (the other way as you have it). Here is the right formula for the doppler_factor: http://www.wolframalpha.com/input/?i=solve+x+%3D+sqrt%28%281%2Bv%2Fc%29%2F%281-v%2Fc%29%29+for+v