Merge this module into a broader category: Math/Algorithms API

I'll probably have time to add a few non-recommender algorithms into the module during the summer, such as Baysian classification, community finding algorithms etc. This is my research focus as a PhD student, so I have the incentive to do it.

However, that kinda switches the module to a more general-purpose direction, and perhaps I'll name it as the Math/Algorithms API module. It'll wrap some Matrix calculation and many other algorithms (maybe w/ existing codes already) too.

As always, the code only provides APIs and functions w/o providing UI. Other developers need to write modules that use it.