IKFast compatibility with sympy-0.7.4

Related to https://github.com/rdiankov/openrave/issues/287

It works for us now, Fedora 20 with sympy-0.7.4, tested with katana-arm, both TranslationDirection5D and Tranform6D (model with a 6th dummy-link).

I couldn't figure out why the Pow._eval_subs method had to be modified for IKFast with sympy-0.7.1, the unmodified version of sympy-0.7.4 works fine. However, I guess you have better knowledge about that and can modify again if necessary.

This might also work with sympy-0.7.3 or sympy-0.7.2, haven't tested it yet.