Default to build with hardening and optimizations

Remove the ENABLE_HARDENING option. If ENABLE_DEVELOPMENT=OFF (which
is default), a release build with optimizations and hardening is
performed. Hardening requries -O2 or higher, which can screw with the
-g flag, which we want when ENABLE_DEVELOPMENT=ON. Therefore, hardening
and development are now mutally exclusive, and hardening comes with
release optimizations.
Fixes #92

Merge branch 'optimization-flag-fix'