Bug fixes and test improvements.

Added

• So many tests, increased coverage by about 10%.

Fixed

• Attempting to import pvl.new without multidict being available,
  will now properly yield an ImportError.
• The dump() and dumps() functions now properly overwritten in pvl.new.
• All encoders that descended from PVLEncoder didn't properly have group_class and
  object_class arguments to their constructors, now they do.
• The char_allowed() function in grammar objects now raises a more useful ValueError
  than just a generic Exception.
• The new collections.PVLMultiDict wasn't correctly inserting Mapping objects with
  the insert_before() and insert_after() methods.
• The token.Token class's __index__() function didn't always properly return an
  index.
• The token.Token class's __float__() function would return int objects if the
  token could be converted to int. Now always returns floats.