new MLlib documentation for optimization, regression and classification

new documentation with tex formulas, hopefully improving usability and reproducibility of the offered MLlib methods.
also did some minor changes in the code for consistency. scala tests pass.

for easier merging, we could maybe rebase these changes (only > feb 7 is relevant) after 
https://github.com/apache/incubator-spark/pull/552
is merged?

jira:
https://spark-project.atlassian.net/browse/MLLIB-19