frameExists is not backwards compatible

[pbeeson]

After moving our DRC robot from groovy to hydro, we experienced tons of issues. This all boiled down to a programmer's frequet use of tf_listener.frameExists(frameID) with a frameID string that contains a forward slash. frameExists, should be smart enough to remove the front slash. I've fixed our code to use the tf::remove_front_sl;ash call, but frameExists should be doing this for the passed string (andin pre-hydro releases, probably the actual frame list as well).