0.9.9 is released

Written for Rapid Photo Downloader by Damon Lynch on 2018-03-08

The purpose of this release is to fix bugs and make life easier for Linux distribution package managers. Specific changes are:

 - Fix bug #1750879: Sequence numbers increment incorrectly for videos when Synchronize RAW+JPEG enabled.

 - Fix bug in sequence letter generation.

 - Enable the disabling of new version checks in both the program logic and the user interface, which is useful for Linux distributions. To disable the check, Linux package maintainers should patch the file constants.py to change the line `disable_version_check = False` to `disable_version_check = True`.

 - Include tornado as a dependency instead of relying on the deprecated mini-tornado found in pyzmq.