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.
@bobvandevijver Yes, you are absolutely right. I didn't think of that!