add standart button styles (btn-success,btn-warning,btn-error)

[bahirul]

Add standart button styles, already test and build with gulp.

[picturepan2]

@bahirul When do you need a green(success) button?

[thehouseplant]

@picturepan2 We tend to use classes like warning, danger, and success for our applications and forms as well. An example would be something like an Edit modal of some kind. In there may be fields that you want to update and, at the bottom, you may have a "success" save button, a neutral cancel button, or a "danger/warn" delete button. Sometimes those visual cues can help prevent a user from selection the wrong option.

In addition to this, sometimes the added colors can help break up an otherwise monotone theme. It may be a simple, but coming from frameworks like Bootstrap and Foundation have also sort of put us in the mindset of having some of those colors available to us when building form interactions.

[picturepan2]

@bahirul I added success and error button colors with the button mixins.