0.9.0

• Added bundled version pytest plugin to base installation.
• Added universal composability for all allowances (using UNION and
  INTERSECTION via "|" and "&" operators).
• Added allowed factory class to simplify allowance imports.
• Changed is_valid() to valid().
• Changed ValidationError to display differences in sorted order.
• Added Python 2 and 3 compatible get_reader() to quickly load
  csv.reader-like interface for Unicode CSV, MS Excel, pandas.DataFrame,
  DBF, etc.
• Added formal order of operations for allowance resolution.
• Added formal predicate object handling.
• Added Sphinx-tabs style docs for clear separation of pytest and
  unittest style examples.
• Changed DataSource to Selector, DataQuery to Query, and DataResult to
  Result.