Fixes and features:

* Implement ``default_if_empty`` argument which will return the default value
  (if specified) if the value is the empty string. (#205)

* Implement ``parse_time_period`` parser for converting time periods like "10m4s"
  into the total number of seconds that represents.

  ::

      >>> from everett.manager import parse_time_period
      >>> parse_time_period("4m")
      240

  (#203)

* Implement ``parse_data_size`` parser for converting values like "40gb" into
  the total number of bytes that represents.

  ::

      >>> from everett.manager import parse_data_size
      >>> parse_time_period("40gb")
      40000000000

  (#204)

* Fix an ``UnboundLocalError`` when using ``automoduleconfig`` and providing a
  Python dotted path to a thing that either kicks up an ``ImportError`` or
  doesn't exist. Now it raises a more helpful error. (#201)