Pysindy

This release adds functionality for performing SINDy with control inputs (SINDYc). It also implements concatenation of feature libraries via the + operator.

Minor changes:
* SINDy constructor properly instantiates `optimizer`, `differentiation_method`, and `feature_library`
* improved discrete time input handling
* Updated README
* Various documentation typos fixed

Fix syntax errors in rst for the previous long description for PyPI.

The main changes in this release are all documentation-related. In particular we added information on the mathematical underpinnings behind the SINDy method in the README and created a new example notebook walking through the math in more detail.

Update the way optimizers are handled

Previously optimizers needed to inherit from our base class `BaseOptimizer`. In this version users can more easily pass in their own optimizers, which are then wrapped in a custom class performing some postprocessing steps. As a result of this change, we have removed the `LASSO` and `ElasticNet` optimizers since their corresponding sklearn objects can now be passed into the `SINDy` object directly.

Minor changes
- Added quiet mode to `SINDy.fit`
- Use L2 regularization by default for `STLSQ`

Changes in this version:

- Optimizers are properly initialized so that copies created by `MultiOutputClassifier` share the same properties
- Added documentation
- Example notebooks conform to PEP8
- A dummy library, `IdentityLibrary` , was implemented so that fully custom features can be used
- Additional unit tests to improve coverage
- Optimizers have `unbias` option to improve performance

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