Fix #143, #144 and visibility issue when using container.

Fixes offset (#143), sizing (#144) and visibility issues when using `container`:

  * `setSize()` uses $element.width() if it's percentage based.
  * `selectPosition()` now takes the container's offset into account 
  * `selectPosition()` shows/hides `$newElement` based on visibility of `$element`'s parent. This is needed when you show/hide divs containing selectpickers, for example in tabs

These changes allow you to use selectpickers in tabs within a modal dialog (you need to call refresh whenever tabs change).