[MLLIB] [WIP] SPARK-1473: Feature selection for high dimensional datasets

The following is implemented: 
1) generic traits for feature selection and filtering
2) trait for feature selection of LabeledPoint with discrete data
3) traits for calculation of contingency table and chi squared
4) class for chi-squared feature selection
5) tests for the above

Needs some optimization in matrix operations.

This request is a try to implement feature selection for MLLIB, the previous work by the issue author @izendejas was not finished (https://issues.apache.org/jira/browse/SPARK-1473). This request is also related to data discretization issues: https://issues.apache.org/jira/browse/SPARK-1303 and https://issues.apache.org/jira/browse/SPARK-1216 that weren't merged.