Added SimpleLinearRegression classifier and some other stuff

1. Fixed parse_csv and added parse_csv_with_labels

2. Added SimpleLinearRegression classifier, code is inspired from https://svn.cms.waikato.ac.nz/svn/weka/trunk/weka/src/main/java/weka/classifiers/functions/SimpleLinearRegression.java Also, added an example showing to use it. This doesn't support nominal attributes for now. To support it, it needs a weight method in DataSet class. I can work on it after some discussion.

3. Added test for SimpleLinearRegression

4. Added failing test case for parse_csv. The regex used to distinguish between string and number has some problem, please fix it. Reference: http://rubular.com/r/ShQUBCaVef