Various fixes

1.  Implement an `into(foo, pyspark.RDD)` operation
2.  Move the `.max` and `.min` methods from Columns to full Table expressions.  TODO: stop calling them scalars, fix schema.
3.  `spark` and `sql` imports by default.  This slows down imports a bit but reduces typing.  TODO: speed up dispatch resolution system
4.  Pretty HTML printing of TableExprs in the IPython notebook (just hooking up Pandas solution)

cc @chdoig