Tipsy positioning

Added the following options for positioning tipsy elements:
1. tipsy-prepend-selector attribute on the primary element. This dictates where the tipsy HTML nodes will be prepended. If no selector is set, tipsy will prepend to document.body (the original behavior).
2. options.overridePosition function. If one exists, will apply the positioning (or any other styling) returned by the function to the generated tipsy HTML node. Priority order: 1. If one doesn't exist, then...
3. tipsy-position-selector attribute on the primary element. If it exists, tipsy will determine the position of its HTML nodes based on the relative position of the selected element. Priority order: 2. If the attribute doesn't exist, then...
4. original tipsy positioning functions as the default with a Priority order of 3