Host documentation on readthedocs

[lukehsiao]

There's a few parts to this task.

• Setup readthedocs so that documentation is auto built
• Fix the import errors with readthedocs
• Go through the code and improve docstrings throughout (this will be ongoing...)

First part is easy, second part will take time.

[lukehsiao]

After mocking the relevant imports, we have:

reading sources... [100%] user/lf_helpers                                                            
WARNING: autodoc: failed to import module 'fonduer.lf_helpers'; the following exception was raised:
Traceback (most recent call last):
  File "/home/lwhsiao/repos/fonduer/.venv/lib/python3.5/site-packages/sphinx/ext/autodoc/importer.py", line 140, in import_module
    __import__(modname)
  File "/home/lwhsiao/repos/fonduer/fonduer/__init__.py", line 10, in <module>
    from fonduer.async_annotations import BatchFeatureAnnotator
  File "/home/lwhsiao/repos/fonduer/fonduer/async_annotations.py", line 15, in <module>
    from fonduer.features.features import get_all_feats
  File "/home/lwhsiao/repos/fonduer/fonduer/features/features.py", line 1, in <module>
    from fonduer.features.content_features import *
  File "/home/lwhsiao/repos/fonduer/fonduer/features/content_features.py", line 7, in <module>
    from fonduer.lf_helpers import get_left_ngrams, get_right_ngrams, tokens_to_ngrams
  File "/home/lwhsiao/repos/fonduer/fonduer/lf_helpers.py", line 12, in <module>
    from fonduer.models import Phrase
  File "/home/lwhsiao/repos/fonduer/fonduer/models/__init__.py", line 1, in <module>
    from fonduer.snorkel.models.candidate import candidate_subclass
  File "/home/lwhsiao/repos/fonduer/fonduer/snorkel/__init__.py", line 3, in <module>
    from .models import Meta
  File "/home/lwhsiao/repos/fonduer/fonduer/snorkel/models/__init__.py", line 32, in <module>
    from .context import Context, Document, Sentence, TemporarySpan, Span
  File "/home/lwhsiao/repos/fonduer/fonduer/snorkel/models/context.py", line 304, in <module>
    class Span(Context, TemporarySpan):
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases