add @2x select arrow, add .no-select, minor bug fixes

- Select dropdown arrows are optmized for high-pixel devices (webkit
only)
- Helper classes are smarter than before.
Now there are a 'no-select' class and 'last-margin', 'last-padding',
'last-border' classes.
- Minor -prefixes improvements
- <a> styles are smarter than before
- Move vendor skype styles to commented author styles