Derivative and integral steps

This is a WIP, I'm putting this here for feedback. In particular, I'm wondering if this is something that belongs in SymPy itself; it was suggested [here](https://groups.google.com/d/msg/sympy/xnNOf69Y7eA/mSyy4G_lBaIJ) that it might not be. In any case, I also didn't approach this problem with any planning, so this code will need a lot of work.

Changes:

- Added steps for some derivatives and integrals, accessible by querying with `diff` or `integrate`
- Added examples ([implicit differentiation](http://sympy-gamma-li.appspot.com/input/?i=diff%28y%28x%29%5E2%20-%205sin%28x%29%2C%20x%29), [arctan integral](http://sympy-gamma-li.appspot.com/input/?i=integrate%28exp%28x%29%20/%20%281%20%2B%20exp%282x%29%29%29))
- Updated SymPy version (not to latest, however)
  - [This commit](https://github.com/sympy/sympy/commit/ecf3dda599d2dc0d3dd8b3d9c31718fb6b6146b7) breaks App Engine, as [subprocess isn't available](https://groups.google.com/forum/?fromgroups=#!topic/google-appengine-python/2TbU9ZQvAx4). I'll probably work around it later by adding a fake `subprocess` module before importing SymPy.

WIP:

- Don't repeat steps that have been done (e.g. in `diff(cot(cot(x)))`, don't show how to do the derivative of `cot` when applying the chain rule).
- Handle more integral cases (especially involving trig)
  - Will probably need a way to provide alternate forms of trig expressions, possibly using rewrite rules
- Handle logarithmic derivatives
- Handle inverse trig derivatives
- Fix up formatting & wording
- Add simplification steps (trig derivatives in particular can get ugly)

References:

- https://groups.google.com/forum/#!msg/sympy/hHB24OHHzO0/MovhGKD7gMAJ
- https://groups.google.com/d/msg/sympy/xnNOf69Y7eA/yElWcrLZ1zsJ
- https://groups.google.com/d/msg/sympy/QLW1OVEz6Fs/rcuzLBTBn3UJ

Screenshots:

![2c535144-2cd9-4e28-8163-c69458d67fa0](https://f.cloud.github.com/assets/327919/257266/de0fbd7a-8c50-11e2-8c0a-fc765c66b891.jpg)
![Screenshot from 2013-03-13 19:44:14](https://f.cloud.github.com/assets/327919/257274/181a4d00-8c51-11e2-8042-7d6029ccb709.png)