fix the issue with using boost::signal with tf

tf recently moved to boost::signal2, so the
effort display needed to be updated too

I made it so that it would conditionally use
boost::signal2 if the tf version is greater than
or equal to 1.11.3

I also fixed some compiler warnings in this code

This code (the effort display) is actually pretty
wicked... I commented on the original pull
request which added it as a built-in display type

closes #700