ikfast: add separate ikfast for sympy-0.7.4

File added as copy of ikfast.py for now. CMakeFiles has been adapted
to include this file (ikfast_sympy0_7_4.py).
ikfast_sympy0_7_4: remove modification of Pow._eval_subs of sympy.power

The code in sympy has changed, so this modification does not work anymore.
Pow._eval_subs of sympy-0.7.4 seems to work for IKFast without any
modification though.
ikfast_sympy0_7_4: adapt to new API for Monomial.__init__
inversekinematics: import correct ikfast module if sympy >= 0.7.4

There is a separate file for that, ikfast_sympy0_7_4.py
inversekinematics: fix checking sympy_version to import correkt ikfast file
ikfast: merge version for sympy-0.7.4 into single ikfast.py file

Deleted ikfast_sympy0_7_4.py file on the way. Also reverted the related
check for sympy version from inversekinematics.py
ikfast.py: expand compatibility of new code down to sympy-0.7.3

It requires the same changes as for sympy-0.7.4, so ikfast should
run with both versions now.
python CMakeLists: remove ikfast_sympy0_7_4.py for install

Those changes have been merged into ikfast.py and that file
has been removed.
Merge pull request #290 from kb0n/sympy_0_7_4

IKFast compatibility with sympy-0.7.4