* src/HAB/Pica/Parser/PicaPlainParser.php (parseField): Allow multiple
  whitespace characters after field tag.

* src/HAB/Pica/Reader/PicaPlainReader.php: Make the parser configurable.
* src/HAB/Pica/Parser/PicaPlainParser.php: Use instance methods instead
  of static methods, implement interface.
* src/HAB/Pica/Parser/PicaPlainParserInterface.php: New interface for
  PicaPlain parsers.

* src/HAB/Pica/Reader/PicaNormReader.php: Handle case where octet buffer
  is empty.

* HAB/Pica/Reader/PicaNormReader.php (open): Skip over preceeding whitespace.