Oh My Fish has been stable for many months, we no longer need the update
instructions on the main README.

Use rawgit to serve assets with correct Content-Type.

- Complete redraw with pixel alignment by design
- Use a image size of 1024 x 1024px
- Base logo alignments on iOS 7 icon grid
- Use background corner rounding of 48px
- Optimize SVG file with svgo tool (minification)

 - Add <Update Instructions>
 - Add <preview available Themes>

* Resolves oh-my-fish/oh-my-fish#116

In order to pro-actively diagnose an issue on someone's computer, a
`omf doctor' command is being added. This command will initially check
errors that could cause a theme to not be loaded, but in the future more
checks are going to be added to prevent issues from being created for
problems that are fixable by the Oh My Fish user alone.

* Run with Travis CI, if commits contain changes to `db/themes`
* When run; fail build if `docs/Themes.md` is no longer up-to-date

* Generates `docs/Themes.md` that can be used
  for preview available themes