Init from the patch in the issue 3198
Formatting fixes
Add __init__.py for sympy/calculus
Add examples to docstring
Add references
Add tests
Autodetect funcs and vars
Test euler_equations interface
Get rid of using numbered_symbols, no integration here
Support lagrangians with higher-order derivatives
Test lagragians with high-order derivatives
Refine docstring
initial sphinx docs for calculus module
Update list of installed modules: sympy.calculus
Update tests list in setup.py: sympy.calculus.tests
Update tests list in setup.py

    The physics.hep tests were missing.
Merge pull request #2431 from skirpichev/3198-euler

Issue 3198: implement Euler-Lagrange equations