Add StackingEstimator for stacked regression/classifier

[weixuanfu]

What does this PR do?

1. Add a transformer, StackingEstimator, to tpot.built_in_operators to replace VotingClassifier for fixing a bug mentioned in the issue Regressor in a VotingClassifier? #457

2. Simplify import codes in export_untils.py

3. Add 5 new unit tests for this new transformer

4. Update 5 unit tests for changes due to this new transformer

Where should the reviewer start?

tpot.built_in_operators

How should this PR be tested?

Any TPOT example and 4 additional unit tests in the PR

Any background context you want to provide?

See #457

What are the relevant issues?

#457

Questions:

• Do the docs need to be updated? No
• Does this PR add new (Python) dependencies? No

[coveralls]

Coverage increased (+0.1%) to 87.018% when pulling 4a07814 on weixuanfu2016:stack_est into 7bea1ee on rhiever:development.

[rhiever]

Looks like we have merge conflicts to sort out again.

[weixuanfu]

Conflicts in this PR is fixed

[coveralls]

Coverage increased (+0.09%) to 87.129% when pulling a79c95a on weixuanfu2016:stack_est into 9948854 on rhiever:development.