ENH: No implementation for sql intersect

[zbrookle]

There's currently no implementation of a set intersection between tables, which seems like it would be needed in the future for compatibility with sql interfaces, for example

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

INTERSECT

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

I would imagine the syntax would be along the lines of (with table being a TableExpr):

table1.intersect(table2)

also with an option for distinct

table1.intersect(table2, distinct=True)