Prototype System class.

- Edited double_pendulum example to use System class.
- Started writing the system class.

This commit is partial. Maybe we can later squash it with other commits.
WIP: Started tests for System class.
Removing default behaviors from System class.
Fix bug with mass spring damper kwarg.
WIP Write most of the System class, with tests.

I didn't run any tests or check for syntax errors :/
specifieds need a lot of work.
Changed the arguments to the generated rhs to support dictionaries.

This is backward incompatible, but could be deprecated. I'm also not sure I
even want the dictionary parsing in the rhs function due to speed concerns. But
this is the logic that can parse the dictionary for the constants and specified
inputs.
Added test for specified dict.
Updated all code to use new dictionary inputs for generate_ode_function.
About to redo System defaults.
Replace calls to find_ with custom methods.
Fix some minor typos.
Not making constants, specifieds, and initial conditions properties.
Finish System class, with passing tests.

* Will only work with KanesMethod for now.
Update double_pendulum example to use System.
Update documentation for System class.
Minor cleanup.
Restore the fast way of parsing specifieds.
Edits to the README.
Merge pull request #81 from pydy/system

Prototype System class.