[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 rebuild *.jar size is   11,615 kB => 11,787 kB after specializing everything under `breeze.linalg` to `Long` as well.
That sounds reasonable?

In the neuro signal analysis I do, data timestamps are often in Long (microseconds), but the only operations are usually +/- so personally, I don't feel extra strong about having Long specialized. However, I was wondering because some things like `OpenAddressHashArray` in `breeze` are already `Long`-specialized