Add functions for statistical analysis in SQL

[jasperjiaguo]

As discussed with @siddharthteotia, consider adding some common statistical analysis methods SQL language.

Few examples:

1. Pearson's coefficient
2. Sampling (bernoulli/stratified)
3. Histogram
4. Entropy
5. Linear regression
6. Logistic regression
7. SVM

[jasperjiaguo]

Designing the one request - multiple (sequential) queries model for statistical functions. Planning to use mini-batch stochastic gradient descent for regression algorithms 2. 3. 4.

[siddharthteotia]

Supporting histogram, entropy like computations could also be potentially useful

[shahharshil46]

Anyone working for supporting Sampling?
Do we know how much effort is it going to be ? will it be few days or weeks?

@jasperjiaguo @siddharthteotia