#524 - Implementation of a model for CCD charge deflection in images, following Antilogus et al (2014)

This branch is ready for review by a wider group I think, although merging the branch is not highly urgent.

On #524 , @danielgruen and I have added a module (`galsim/cdmodel.py`) to describe charge deflection in CCD pixels, also known as the "brighter-fatter" effect, following the model of Antilogus et al (2014), see http://arxiv.org/abs/1402.0725.

The effect is applied to images at the C++ layer via a new `BaseImage` member function, `applyCD()`.  We chose the constness etc. settings in the way we thought made sense, but comments welcomed.  The Python module, and the classes which store the different charge deflection models, then use this interface to apply the effect.

@danielgruen has put some unit tests in place, and is already using the code to perform tests of the effect's impact on weak lensing measurements.  However, as ever, suggestions for improved docstrings and unit tests would be welcomed on this PR.
```
Time taken to setup (n = 2) PowerLawCD model = 0.001608s
Time taken to setup (n = 3) PowerLawCD model = 0.003806s
Time taken to setup (n = 4) PowerLawCD model = 0.007210s
Time taken to setup (n = 5) PowerLawCD model = 0.010473s
Time taken to setup (n = 6) PowerLawCD model = 0.015573s
Time taken to setup (n = 7) PowerLawCD model = 0.020352s
Time taken to setup (n = 8) PowerLawCD model = 0.026478s
Time taken to setup (n = 9) PowerLawCD model = 0.034254s
```
@danielgruen tells me that for DES the proposed correction is n = 5, so this is taking 1/100th of a second in that one expected use scenario.  I think that's minimal given that we expect to use one PowerLawCD model per DES chip, and (I belive) only set this up once then simply re-use it N times.  But if that way of using the code were to change it would be worth speeding this up.