refactor xapers.parser.ParserBase

This renames the class to DocParser and
changes its signature in that its methods now work
on paths directly. This way, one needs only one DocParser
for every mimetype.
remove superflous implementation for ParseError

which replicated the default behaviour. This version
is semantically equivalent.
add SubprocessDocParser

This is a generic document parser that works by calling
external "doc2text" commands as given as parameter
in its constructor
Replace PDFParser with SubprocessDocParser
remove unused xapers.parsers.pdf
use ps2txt to parse postscript files