pandas: enforce expression column names on by
expr: column name property
expr: name of column + scalar -> is name of column
pandas: rename apply columns pre-join

This avoids naming overlaps when column names are the same in the
grouper and apply sections of a By operation

Thanks to @talumbau for the test and issue

Fixes https://github.com/ContinuumIO/blaze/issues/468
add comments around dataframe column renaming
Merge pull request #459 from mrocklin/pandas-groupby-naming

pandas: enforce expression column names on `by`