[Incomplete] Document Data Structures

Hi David,

Sorry for the sudden hiatus... work was killing me
I'd like to gradually add some of the code I've written for work into breeze and get into contributing again.
For that, I think I really should understand the inheritance structure of DenseVector and DenseMatrix.

I've started an ambitious attempt to take notes and make a UML-ish diagram... do you think something like the following would be useful to really make complete?
![image](https://cloud.githubusercontent.com/assets/4279323/3775458/419e2098-1945-11e4-9190-1a6eb0bbc064.png)

I'd like to go through the classes and add more docs with your help, as well as make the variable/type anno naming consistent, if that's OK with you?

Sincerely,
Kenta
The idea was to be analogous to the collections library, with Seq and SeqLike. Seq needs to inherit from SeqLike (for the return type annotations to match the particular kind of Seq) and the other classes need to inherit from both. 

But, as we've moved away from methods and towards UFuncs, this has become less and less useful. Probably the Likes can be gotten rid of, though classes will have to inherit from NumericOps directly, which looks like the one thing the Likes are actually doing right now.