Fixing warnings.

Refs #1777.
Fixing overloaded virtual warning.

@friedmud, I think the threadJoin() function prototype was wrong,
it took a UserObject reference before... this was also part of the
warning and is addressed by the patch.

Refs #1777.
Merge pull request #3164 from jwpeterson/warnings

Fixing warnings.
Merge commit '598c47336d75f307cb4df4a9ebf25329a8448819'