stat_holder now maintain the steps itself, to work better with the step callbacks which produce stats.

* initial public refactor

* add uniform

* simplify code

* split uniform-factor

* fix prior

* working version

* more documentation

* clean up done for InfoGAN

* updated other examples to match changes in GAN.py

* final edits from my side

* docs changes in base class

* more docs and interface name

* some changes in name scope and docs

* use property for _dim

* visualization & gradient scale according to the paper

* update the demo img