Add padding attributes for background fill

Normally the background fill is applied directly around the glyphs. These changes add the option to apply a horizontal and vertical padding to the background fill, so that one can customize it to look nicer.
Change padding to single UIEdgeInsets attribute
Merge pull request #243 from iOctocat/master

Add padding attributes for background fill
Set paragraph styles on mutableActiveLinkAttributes
Bumping version to 1.7.1