Backport a few fixes for pip 6.0.1

* Closes #2248 - Ensure that we preserve executable file permission bits
* Closes #2247 - Fix import so exception module is accessible.
* Closes #2250 - Fix a typo in the change log.
* Closes #2252 - Fix the missing list of versions when no version matching specifier can be found.
* Closes #2253 - Warn about `>` being an exclusive operator now.
* Closes #2257 - Revert "Speed up unpack_file_url".