Added mint theme based on united theme.

1. Gruntfile.js added mint to swatch task
2. Variables.less replaced @orange color with @darkGreen color.
3. Index.html changed Title from United to Mint and description from Ubuntu orange to Ubuntu green.

All another files are copies from united theme.
Added mint theme based on united theme.

1. Gruntfile.js added mint to swatch task
2. Variables.less replaced @orange color with @darkGreen color.
3. Index.html changed Title from United to Mint and description from Ubuntu orange to Ubuntu green.

All another files are copies from united theme.