Linear regression

An implementation of linear regression, accompanied by unit tests & benchmarks.

Includes some test data  *(exams.csv / exam.csv)* from [here](http://college.cengage.com/mathematics/brase/understandable_statistics/7e/students/datasets/mlr/frames/frame.html)

Tested & compared results using the online calculator available [here](http://www.xuru.org/rt/MLR.asp#CopyPaste) and results seem to match. Meaning, either my implementation is correct or I have re-implemented the same bugs as xuru.org... :smile: 

It's also reasonably fast. Single-threaded on my machine:

    BenchmarkLinearRegressionOneRow	 5000000	       326 ns/op