[SQL] DataFrame API update

1. Added Java-friendly version of the expression operators (i.e. gt, geq)
2. Added JavaDoc for most operators
3. Simplified expression operators by having only one version of the function (that accepts Any). Previously we had two methods for each expression operator, one accepting Any and another accepting Column.
4. agg function now accepts varargs of (String, String).

Author: Reynold Xin <rxin@databricks.com>

Closes #4332 from rxin/df-update and squashes the following commits:

ab0aa69 [Reynold Xin] Added Java friendly expression methods. Added JavaDoc. For each expression operator, have only one version of the function (that accepts Any). Previously we had two methods for each expression operator, one accepting Any and another accepting Column.
576d07a [Reynold Xin] random commit.