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.
Minor refactoring of link setting
Adjusting label width
Bumping version to 1.7.4
Revert "[Issue #274] Resetting links on -setAttributedText:"

This reverts commit 39da66694fcd41d86e411ba934655e5cdbd0039a.

Conflicts:
	TTTAttributedLabel/TTTAttributedLabel.m
Bumping version to 1.7.5