Merge tag '0.8.0' into develop

0.8.0
Add gameplay movie.
Add link to asciinema.
Updated the how to play section.
Update screenshot.
Add gitter chat badge.
Flat gem version badge.
flat Travis badge.
flat codeclimate badge.
Flat coverage badge.
flat gemnasium badge.
Flat gittip badge.
Flat gitter badge.
Integrate with gitter.
Choose blue for the gitter badge.
Refactor.
High score.
Extract Text2048::HighScore class.
Fix CI error.
Add title screen.