Protect self.links for multiple additions of links.

When calling -addLinkToURL:withRange: multiple times, only the last add
is maintained. The underlaying cause is that self.links array is reset
when self.attributedText is set in the method's if (attributes) body.
The alternative fix was to protect the self.links property in the
setAttributedText.
Merge pull request #280 from adonoho/adonoho-self.links-fix

Protect self.links for multiple additions of links.