Fix some C++ compiler warnings

[ritwit]

Addresses a part of Issue #74

Warnings that are addressed by this PR:

• -Wsign-compare
• -Wunused-variable
• -Wunused-but-set-variable
• -Wmaybe-uninitialized (probably the most important)
• -Wunused-parameter (from -Wextra)
• -Wstrict-prototypes (enabled by setup.py but not valid for C++)

I may be able to look at other warnings too if I get time. Thanks.