Include a className of 'is-clearable' if clearable is true

Author: DDiaz

src/Select.js

@@ -1079,6 +1079,7 @@ const Select = React.createClass({
 		let className = classNames('Select', this.props.className, {
 			'Select--multi': this.props.multi,
 			'Select--single': !this.props.multi,
+      'is-clearable': this.props.clearable,
 			'is-disabled': this.props.disabled,
 			'is-focused': this.state.isFocused,
 			'is-loading': this.props.isLoading,