Get rid of size_t warning on vs2013 for lepton.

A few weeks ago, the OpenSim people upgraded to the latest lepton from this repo. Then we started getting warnings in VS2013 (see https://github.com/opensim-org/opensim-core/issues/39). This PR intends to get rid of those warnings. If this gets merged in, then at OpenSim, we'll upgrade our lepton again to the latest from this repo.