[WIP] Linearize lagrange

This is functioning (and almost done) code for linearizing Lagrange's equations.

TODO:
- [x] Add deprecation warning for removal of `coneqs` kwarg from LagrangesMethod
- [x] Add another test (Rolling disc?)

Already done:
- Modified the `Linearizer` class to work with Lagranges Method. This required a reformulation of the general form. See [here](http://jcrist.github.io/gsoc-week-4.html) for details.
- Redid the interface of LagrangesMethod to include both the holonomic (not time differentiated), and non-holonomic constraints. These are needed for linearization.
- Added `to_linearizer` and 'linearize' class methods to match those in KanesMethod
- Added a `solve_multipliers` method, that solves for the value of the multipliers at a given operating point, and returns a dictionary of {multiplier: value} pairs.
- Two tests are in place; one for a minimal pendulum representation, and one for a non-minimal pendulum.
@jcrist I'd like to have this as a publicly available function. I'm always in need of this when manipulating expressions.