v0.4.0

- Change `sortable.init` to `sortable.initTable` and make `sortable.init` initialize all tables with `data-sortable` attribute
- Simplify `data-sortable="true"` into data-sortable since its presence can be taken 
- Prevent double initialization using `data-sortable-initialized` attribute (Closes #2)
- Change `th[data-sort="false"]` to `th[data-sortable="false"]`