BUG: Fix for multiple ints as levels in DataFrame.stack()

Fix for multiple level numbers

If passed levels match level names, use them directly

Add test cases for multiple stacking

Add check for out of range negative level num in _get_level_number()

Use _get_level_number() to do validation and conversion

Explain why level list isn't iterated over directly

Raise exception on fall-through case

Add test cases for raising exceptions

Use _get_level_number() to convert and validate negative levels

Add changes to release notes

Fix Python 2.6 build issue: use assertRaisesRegexp from testing module

Add tests for out of bounds level numbers

Move API change note to correct section

Add blank line after if/elifs

Add blank line between sections

Too many blank lines between functions

Add multiple stacking examples

Reference to new examples in What's new docs

More blank lines between sections

Remove unused itertools import
Merge pull request #7770 from onesandzeroes/int-stacking

BUG: Fix for passing multiple ints as levels in DataFrame.stack() (#7660)