add macro support

- when loading a corpus, a macros file can be specified:
  ``c = CorpusReader('corpus.dact', macrosFilename='macros.txt')``
- macros will be expanded when doing xpath queries:
  ``c.xpath('//node[%headrel%]')``
improve macro support

- make ``macros`` member a pointer
- expand macros in ``xpathWithStylesheet`` function as well
- store expanded query string in temporary std::string
- reformat docstring
- restore ``-mt`` postfix in ``setup.py``
Merge pull request #3 from andreasvc/master

Add macro support.