Fix and refactor SASS compilation

- SASS files are now copied to temp dir before modifying variables
- Compilation is done in StylesheetCompiler
- CommunityStylesheetCompiler is community-aware class that utilizes
  StylesheetCompiler to compile community specific custom stylesheets
Add new image folder in public/system to ignore
Merge pull request #223 from sharetribe/fix-css-temp-file-issue

Fix and refactor SASS compilation
new translations
Fixed a bug that caused user name to be shown wrong in listed & grid listings
How to use page can be edited with Mercury
Custom translations
UTF-8 -proof downcasing for global header links
Fixed conflict