Fix for next tab

Javascript fix for next tab setup.

Thanks to geckopeb & sebastianmenendez

CSS cleanup and small visual updates

Moved all CSS under .wizard to avoid class name conflicts. Small visual updates include:

• No hover on disabled steps
• More reliable active triangle centering using CSS translate.

Set minimal Yii2 version to 2.0.6

Bugfix completed step

Completed step becoming active when last step is selected is fixed.

Button HTML option

Configuration option to specify button with HTML code. Completed documentation with configuration explanation.

Fix dependency withYiiAsset

Fix for report http://www.yiiframework.com/extension/yii2-wizardwidget/#c19639

Icon spacing & example fix

Fixed spacing of the step icons by calculating explicit the percentage of each icon based on steps+complete. Fixed mistake in example within readme.md.

Start step to use array index

Start step determination is now compliant to use non integer step ID's. It's also possible to use 'completed' as start step. Plus added visual clue if step is not available.

First stable release

Stable API and complete feature package to make it usable in a production environment.