Lua compatibility

[jprjr]

Split out from #97

Fixes a few bugs with Lua compatibility.

• Lua 5.2 introduced changes to tonumber which caused affected_rows to always be nil
• Pgmoon spec tests use os.execute and checked for an exit code of zero, Lua 5.2 returns multiple values, the exit code is now the last value.

Adds a new bit module for compatibility with Lua 5.3's bitwise operators, and associated spec tests to ensure same behavior as luabitop.

[leafo]

Thanks