Mocha.jl v0.0.6

[Mocha.jl](https://github.com/pluskid/Mocha.jl) v0.0.6: unsupervised pre-training with stacked auto-encoders.

## v0.0.6 2014.12.31

* Infrastructure
  * Numerical gradient checking in unit-tests (@pcmoritz)
  * Simple ref-counting for shared parameters
* Network
  * RandomMaskLayer, TiedInnerProductLayer, IdentityLayer
  * Freezing / unfreezing some layers of a network to allow layer-wise pre-training
* Documentation
  * A new tutorial on MNIST that compares unsupervised pre-training via stacked denoising auto-encoders and random initialization