Take control over the CSS and simplify the directives.
- Drop jquery.tagit.css. It's easy enough to style ourselves and most
  of it comes from the ui theme anyway.
- Simplify the directive.
  - Don't swap between editor and viewer widgets. Manage the disabled
    state, placeholder, and styling differences ourselves.
  - Stop creating a new scope. This simplifies the model binding by
    avoiding the $parent eyesore used in the annotation. This was
    accomplished by observing the readonly attribute instead of watching
    a scope variable.
  - Use the <ul> form of tagit rather than the input. The need for a
    template is completely removed.
- Allow underscores in tag names.