Documentation Revamp

This PR includes the following changes:
 * Refactored some sections into their own files at doc/ (Overview, Convolutions, Transfer functions, Module, Criterions, Simple Layers, Containers, etc.). This makes them easier to edit on github (the README was lagging in edit mode).
 * Refactored images into their own directory (doc/images)
 * Fixed changes in the relative links due to the above changes.
 * README is basically an index to these files with small descriptions. This is harmonized with the [Torch](https://github.com/torch/torch7/blob/master/README.md) package.
 * Divided Convolution into different sections : Temporal, Spatial and Volumetric.
 * Added an index to modules in convolution.md with short descriptions for quicker access.
 * Added an index to containes in containers.md and a section linking to Table layers (which has some containers).