Test some basic properties of iterable()
Updated finite_diff according to feedback.

* finite_diff is moved from sympy.series to sympy.calculus
* Derivative.as_finite_diff got a new kwarg: ``wrt`` for use on partial
    derivatives, the arguments indep_vals and around were also renamed
    to points and x0 respectively.
* Derivative.as_finite_diff was refactored and made PEP8 compliant
* finite_diff.py and test_finite_diff.py were made PEP8 compliant
* Typos were fixed througout the docstrings
* A small part on finite differences were added to the tutorial on calculus.