porposal to fix $this->outputDir is NULL

Hi,

I added configurable outputDirectory and additional options for pdflatex.

This commit may be a way to fix #5, but I breaks backwards compatibility for LatexGenerator::generatePdf(). I don't think this is a real issue, because it did not work at all before.
@kryptychon This line is really crap: here you change the output directory of the newly generated tex file to something else, after which you can no longer create a PDF from the given LatexInterface. The generateLatex() function also generates the output directory, so this new behaviour is never desirable.