Quantile regression

* [x] Warning about cannot overwrite scale attribute
* [x] Summary function (e.g. pseudo R2)
* [x] what about mse,rsquared and all the other linear_model results? Do they make sense?
* [x] Test for different quantiles (maybe only for a subset of kernels and bandwidths)
* [x] Parzen kernel does not work
* [x] Gaussian kernel does not work
* [x] Cosine kernel does not work
* [x] Biweight kernel does not work
* [x] Which other attributes to test?

quantile regression as discussed in this thread: 

https://groups.google.com/forum/?fromgroups=#!topic/pystatsmodels/vfKh368aPkU