Improvements

Changes:

- New dispatch/evaluation mechanism for cards
    - Finds result sets both by name of top-level function and type of evaluated expression
    - Can show cards from multiple applicable sets
- Can [change the variable](http://13.sympy-gamma-li.appspot.com/input/?i=diff%28cot%28xy%29%2C%20y%29) of a results page when multiple variables exist
- [Decimal approximations everywhere](http://13.sympy-gamma-li.appspot.com/input/?i=solve%28x**2%20%2B%204*x%20%2B%20181%2C%20x%29)
- [`/random` URL](http://13.sympy-gamma-li.appspot.com/random)
- [Synonyms](http://13.sympy-gamma-li.appspot.com/input/?i=antiderivative%28cos%28x%29+-+sin%28x%29%29) (only a few currently)
- Special formatting for input: `prime`, `isprime`, `factorint`, `factor`, `plot`, `nextprime`, `solve_poly_system`
- Initial 'Did you mean?' feature (only works for [factor/factorint](http://13.sympy-gamma-li.appspot.com/input/?i=factor%28100%29) currently, though in this case it's redundant because of the first change listed)
- New result sets:
    - [`plot`, `graph`, or `draw`](http://13.sympy-gamma-li.appspot.com/input/?i=plot%28x^2%29)
    - Matrices: inverse, eigenvalues/eigenvectors. 
- New result cards:
    - Polynomial with this root
- Modularized HTML templates
- Misc. CSS/JS fixes

Demo: http://13.sympy-gamma-li.appspot.com/

For matrices, I'd like a syntax where `determinant([[1,2],[3,4]])` will get translated to `Matrix([[1,2],[3,4]]).det()` so the user can use intuitive math notation but still see the SymPy equivalent. I can achieve the translation with AST transformations (doing it at the symbol level would be tedious, and at that point we may as well implement a custom parser), but then we'd need a way to translate the AST nodes back into Python. I think `unparser` in the Python source can achieve this.

Todo:
- [x] Don't show redundant approximation (http://13.sympy-gamma-li.appspot.com/input/?i=E)
- [x] Square viewport/reset buggy (http://13.sympy-gamma-li.appspot.com/input/?i=plot%28x^2%29)
- [x] `root_to_polynomial` is not showing
- [ ] Add new examples