Remove bitwise ops, min, max and abs from fun.operator. Close #5.

This change removes worthless shortcuts to bit and math module from
fun.operator. Please use appropriate modules directly.
Fix invalid package.path in tests/runtest
Use modulo operator instead of math.fmod for operator.mod

This commit changes semantics of operator.mod to return results with
the same sign as its second operand (divisor).
Add support for Lua 5.1 - 5.3 in test suite
Mention Lua 5.1-5.3 in the documentation