[bug] fix lookbehind assertion fails with /m mode enabled (Issue #15)

Optimization for .* should be disabled if lookbehind assertion is used.

see: http://bugs.ruby-lang.org/issues/8023
[dist] update testpy.py

Add tests for the Issue #15.
update HISTORY
Merge branch 'topic/fix-lookbehind-with-dotall-mode'