Keris

- cythonized im2col operation
- optimized im2col operation for 1x1 convolution
- removed double precision support
- added 2 default callbacks
- fixed backward computation of softmax layer
- added binary_softmax_crossentropy, now softmax_crossentropy renamed to categorical_softmax_crossentropy

- Added support for callback
- Removed metric history, soon will be replaced with callback
- Fixed files outside package directory