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
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
Fine-tuned new dropdown field form
How to use page can be edited with Mercury
Custom translations
UTF-8 -proof downcasing for global header links
Merge branch 'master' into develop

Conflicts:
	config/locales/en.yml
Custom field validations

Conflicts:
	app/models/custom_field.rb
Show error if saving fails
Validates locale name
Validate at least one category
Validate options
Fixes to make things work with earlier rename
Merge with develop
rename selected_option class too