Determine and fix root causes of all build warnings on all platforms

[shasheene]

After #7 is merged, it's probably a good time to triage and resolve all build warnings.

With compiler upgrades (eg, compiling with C++11 or C++14), new warnings get generated.

Few warnings occur on Ubuntu ubuntutrusty.travis-ci.org.jobs.200830534.log.txt.

Some occur on the macOS build macos.travis-ci.org.jobs.200830541.log.txt

Huge amounts occur on the Windows build with VC++ appveyor_log.txt [Warning: Due to OGRE build, log file is 9MB in size]

Task is to produce a PR that solves all of them (and likely learn bunch more about C++ standards in the process) Once again, may be months before I get around to generating a PR.