Fix error when append html starts with white space

Because of security-motivated changes to $(html) in jQuery 1.9, if the ajax response for more content begins with any character other than < (even if there are only spaces and/or carriage returns before the first <), waypoints-infinite will cause jquery to throw "SYNTAX ERROR, UNRECOGNIZED EXPRESSION".

This change fixes the problem by implementing the new best practice described here:
http://jquery.com/upgrade-guide/1.9/#jquery-htmlstring-versus-jquery-selectorstring

Also see:

http://bugs.jquery.com/ticket/13223