fix :focus state in mixin buttonBackground

&:focus is missing on line 502, so buttons look broken on focus
Merge pull request #55 from flachware/fix-buttonBackground-sass-twitter-bootstrap

fix :focus state in mixin buttonBackground
update javascript files to v2.3.1
update docs to v2.3.1
update documentation to v2.3.1
update built css files and documentation
customize composer.json
Merge pull request #57 from greg0ire/composer

customize composer.json
Incoming package on packagist.org
@jlong: thanks for your trust
document Composer installation
Merge pull request #56 from roverwolf/2.3.1-updates

2.3.1 updates
prefix sass section in readme