Rename `media_types_and_character_encodings` dir

Change `media_types_and_character_encodings` directory name to
the shorter `media_types` as character encodings are part of the
media types.

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From http://en.wikipedia.org/wiki/Internet_media_type:

 "A media type is composed of a type, a subtype, and zero or more
  optional parameters. As an example, an HTML file might be designated
  `text/html; charset=UTF-8`. In this example `text` is the type,
  `html` is the subtype, and `charset=UTF-8` is an optional parameter
  indicating the character encoding."
Improve media type grouping consistency