Experimental Light theme

This is an experimental light theme written by @simonhaenisch
and published originally in

http://github.com/AlpenglowTheme/alpenglow-theme/issues/17#issuecomment-220962908
  and
http://github.com/AlpenglowTheme/alpenglow-theme/files/277426/Alpenglow.Light.zipe:

Further TODOs:

- [ ] create tab bar png files (to highlight dirty tabs)
- [ ] set layer textures for tab bar
- [ ] create darkened icons
- [ ] test all elements/theme settings

![screenshot with solarized light scheme](https://cloud.githubusercontent.com/assets/3410759/15470266/eb7905ca-20ef-11e6-8dba-3cc63c7b45c7.PNG)