FED-106 Backplane2 /messages throws ParseException when no "since" param

Check to make sure the since parameter isn't blank -- null, empty string, or whitespace
Re-doing this commit on the prod branch per traney
Also make sure the change is actually on the backplane2 code
Merge pull request #27 from janrain/steven2012.46_RC2/FED-106

FED-106 Backplane2 /messages throws ParseException when no "since" param