make button_personality an extendable %placeholder
convert @include button_personality to @extend, remove @include, whitespace cleanup
enable legacy support of @include button_personality
Merge branch 'master' into extend-button

Conflicts:
	_site/assets/css/sassquatch.css
	_site/assets/css/sassquatch_mobile.css
	_site/assets/css/sassquatch_tests.css
	css/sassquatch.css
	css/sassquatch_mobile.css
	jekyll_docs/assets/css/sassquatch.css
	jekyll_docs/assets/css/sassquatch_mobile.css
	jekyll_docs/assets/css/sassquatch_tests.css
	sass/components/_navbuttons.scss
	sass/core/_forms.scss
	sass/util/_mixins.scss
Merge pull request #134 from meetup/extend-button

@extend button_personality