[WIP] Fix `pow` behavior on negative numbers

[castarco]

Currently, making $a->pow($b) can produce errors if $a is a negative value. To fix the problem it's a good idea to solve the bugs #21 and #22 .

[castarco]

Partly done (negative numbers with integer exponents behave well), now depends on #22 (the exponent must be understood as if it's a rational number, check the divisor, and if it's even then throw an exception, but if it's odd, the pow can be computed).