Fix most warnings generated from -Wmost, for SimTKcommon.

Addresses #181.

See travis output for the remaining warnings. So far, I only addressed warnings from SimTKcommon. SimTKcommon still has two warnings, but I don't know what to do with them.
If our objective is to not generate all those warnings, then I think we should put that in travis. Otherwise, we'll have to do another cleanup in the future.