Patch: at_revision and at_time buxfixes

Bugfixes for Document.at_revision() and .at_time() methods:

• convert filter to list (python3)
• apply each revision separately due to depreciated support for list in Revision.apply()

Version 1.0.0

• Uses google-auth instead of oauth2client for google credentials (#6). See https://google-auth.readthedocs.io/en/latest/oauth2client-deprecation.html) for oauth2client depreciation notes.
• Removes suboperation class (speed up document parsing and reduce memory usage)
• Supports google docs using "suggestions" (track changes) (#7)
• Python 3 support (#8)
• Utilities for measuring GoogleDoc initialization running times (#9)
• Adds tests (#10)

Version 0.5

• Memory optimization with python __slots__
• Undo functionality
• Optimized replay class
• Measure elapsed times for Document methods

Version 0.4

• Introduces Suboperations for tracking single-character (or element) changes
• Single Content.apply() method replaces Content.apply_operation(), Content.apply_revisions(), etc

Version 0.3

• Bugfix for docs with footnotes, by including EndOfBody dummy element type for separating document body from footnote text
• Create Character instances on insert operation initialization, rather than on apply
• Easier document initialization - support specifying service account keyfile location as opposed to creating and passing a credentials object to
• iter_operation methods on document/revision/operation objects for flattening operation tree and easier iteration through operations

Version 0.2

Changes from version 0.1:

• logging debug messages
• suppress warnings from google api client library
• replay iterator method on Documents
• documentation with Sphinx
• bugfix for listing revisions for blank documents

Version 0.1

Initial beta version of gdocrevision library.