Add a "Copy Comment" feature

- Adding feature request #92.
- Pressing the action menu button now shows a "copy comment" button
that copies comment body text to the iOS system clipboard.
- Added method (found at
http://stackoverflow.com/questions/6170531/strip-out-html-tags-etc-from-nsstring) 
to strip html tags from comment body text for copying.
- May need refactoring

As this is my first foray into iOS or Objective-C, this patch may need some refactoring but I think it provides a good starting point.