Add CSS3 selection mixin

Selection mixin explained/requested in issue #542.
Add @import to main bourbon partial for selection mixin
Add @import to /dist bourbon partial for selection mixin
Simplify selection mixin
Merge pull request #544 from Bionicrm/master

Add CSS3 selection mixin
Change logo in README to SVG
flat button mixin should consider padding and textsize arguments
Deprecate button mixin
Remove dist directory and repoint Bower
Closes #480
Depricate grid functions that are now in Neat
Updated Modular Scale function to have defaults
Make golden ratio function compatible with update