Replacing ^ as safe_power does not consider the proper order of operation

[roniemartinez]

We used to use cexprtk which treats ^ as power operation. As we converted to simpleeval which treats this as bitwise XOR, we had to put the change advised in the README.

>>> import ast
>>> from simpleeval import safe_power

>>> s = SimpleEval()
>>> s.operators[ast.BitXor] = safe_power

>>> s.eval("3 ^ 2")
9

Unfortunately this does not change the order of operation. Below is an example (both should result into 6):

>>> s.eval("5**1+1")
6
>>> s.eval("5^1+1")
25