- DocTest is now a simple container class; its constructor is no longer

responsible for parsing the string.
- Renamed Parser to DocTestParser
- DocTestParser.get_*() now accept the string & name as command-line
  arguments; the parser's constructor is now empty.
- Added DocTestParser.get_doctest() method
- Replaced "doctest_factory" argument to DocTestFinder with a "parser"
  argument (takes a DocTestParser).
- Changed _tag_msg to take an indentation string argument.