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? 

@wkerzendorf, If I put v = c, in the formula for z, then I get:
z = sqrt[(1 + c/c)/(1 - c/c)] - 1
=> z = sqrt[2/0] - 1
=> undefined

In fact, if I use the formula you have given for doppler factor:
v = c * [(df**2 - 1)/ (df**2 + 1)] 
Since df = z + 1, substitution gives me the same formula I have used. I have also tested the code, the formula seems correct. 

Anyway, I guess logically it makes more sense to use the formula for doppler_factor, so I will reverse the code as you suggested