sklearn compatible hyperopt_estimator constructor

[mgbckr]

Referring to #168, there is an issue with sklearn's get_params method called on HyperoptEstimator since HyperoptEstimator does no adhere to the the constructor guidelines of sklearn. In particular, the constructor (__init__) should copy all constructor arguments to attributes of the same name and not contain any logic.

This pull request fixes this issue by pulling the constructor logic into a new _init method and calling that method only in fit.

[Aaditya-Bhatia]

Tested on Ubuntu with Anaconda. Works for me, the bug is resolved.
Thanks!

[mandjevant]

Two issues with this:

1. _init() would run twice if fitting was done twice. Example test case
2. Initiation logic would be skipped if fit_iter() was called instead of fit(). Example test case