Added dispatcher for select objects

* In pytest_selenium fixture, select_by_visible_text() and select_by_value
  have been updated to receive a select element, rather than a locator. Just
  as have their deselect counterparts.
  This is required because of the introduction of the DHTMLSelect() object.
* A new Select() object has been created which subclasses the Selenium Select()
  object. This, along with changes to DHTMLSelect() mean that their methods
  are identical and hence the select() function in pytest_selenium can act on
  either element.
* paginator module has been updated to use the new Select() object.
* Multiple files have been updated to use the new Select() object in the Form
  definitions.
Merge pull request #515 from psav/dhtml_select_dispatch

Added dispatcher for selects