Adding support for standard sharing panel for Issue #19

First pass at answering issue #19.

One issue with this patch is that adding UIDocumentInteractionController duplicates a bunch of the options on the action menu already (printing / emailing / posting to Twitter / posting to Facebook). They could probably be removed, although you'd lose the ability to set things like initial text on Twitter postings and subject on emails because UIDocumentInteractionController doesn't seem to allow that.

Please let me know if you'd like an updated patch removing the duplicated options. I'll happily update the code.