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 know how the rest of the code is organized. However, the `DopplerWCS` should not take a WCS as an input, but just a number which it multiplies with. So the `__call__`-function should be:


```python
def __call__(self, dispersion):
    return dispersion / (1 + self.doppler_factor)
```

In fact the `__init__`-function should take a velocity and convert it to a doppler factor  :

```python

def __init__(self, doppler_factor):
    self.doppler_factor = doppler_factor
```

and add some class methods to initialize with different inputs such as velocity (see http://en.wikipedia.org/wiki/Relativistic_Doppler_effect) and z.

```
@classmethod
def from_velocity(cls, velocity):
    beta = velocity/constants.c
    doppler_factor = sqrt((1+beta)/(1-beta)
    return cls(doppler_factor)
```

```python
@classmethod
def from_z(cls, z):
   return cls(z+1)
```