Rolled light and dark themes into one file

- The highlighting function now expects an array for its color
  arguments. The first value in the array is the color to use for the
  dark version of the theme, the second number is light.
- Everything has been set up to switch correctly on set bg=light/dark.
  The theme queries &background and makes color choices appropriately.
  Vim will re-run coloring when the value of &background changes.
- The light theme file has been removed.
Removed an old call to set 'background'
Merge two versions with backward compatibility
Implement complete test scenario for every state transition
Revert DiffChange background color
Change array formatting
Update MatchParen background
Fix typo
Fix state transitions. Force seoul256-light to choose the light version.

Fix errors on:
- dark g:seoul256_background + set background=light
- light g:seoul256_background + set background=dark
Choose DiffChange background according to g:seoul256_background
Update screenshot
Add link to seoul256-iTerm2
Add screenshot for all g:seoul256_background values
Add .travis.yml
Ignore error on test case for travis-ci
Vim output to /dev/null
Introduce g:seoul256_light_background (#7)
Update version/last updated tags
Add travis-ci badge
Remove vim-scroll-position colors