CLN/ENH: pep8 junk and unicode string formatting for error messages
TST/CLN: separate out inner and outer join tests
CLN/ENH: refactor assertion for periodindex joins
CLN: raise the correct kind of exception
CLN/ENH: refactor and disallow certain types for period and datetime index joins
TST: add tests and fix up failing to match new behavior
TST/BUG: nose assert_is_instance does not work on python26