Add an `explode` function for dataframes and modify the analyzer so that single table generating functions can be present in a select clause along with other expressions.   There are currently the following restrictions:
 - only top level TGFs are allowed (i.e. no `select(explode('list) + 1)`)
 - only one may be present in a single select to avoid potentially confusing implicit Cartesian products.

TODO:
 - [ ] Python

Author: Michael Armbrust <michael@databricks.com>

Closes #6107 from marmbrus/explodeFunction and squashes the following commits:

7ee2c87 [Michael Armbrust] whitespace
6f80ba3 [Michael Armbrust] Update dataframe.py
c176c89 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into explodeFunction
81b5da3 [Michael Armbrust] style
d3faa05 [Michael Armbrust] fix self join case
f9e1e3e [Michael Armbrust] fix python, add since
4f0d0a9 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into explodeFunction
e710fe4 [Michael Armbrust] add java and python
52ca0dc [Michael Armbrust] [SPARK-7548][SQL] Add explode function for dataframes.