[SPARK-6129] [MLLIB] [DOCS] Added user guide for evaluation metrics

Author: sethah <seth.hendrickson16@gmail.com>

Closes #7655 from sethah/Working_on_6129 and squashes the following commits:

253db2d [sethah] removed number formatting from example code
b769cab [sethah] rewording threshold section
d5dad4d [sethah] adding some explanations of concepts to the eval metrics user guide
3a61ff9 [sethah] Removing unnecessary latex commands from metrics guide
c9dd058 [sethah] Cleaning up and formatting metrics user guide section
6f31c21 [sethah] All example code for metrics section done
98813fe [sethah] Most java and python example code added. Further latex formatting
53a24fc [sethah] Adding documentations of metrics for ML algorithms to user guide