Added the pdf backend.
Changes to the way the pdf backend is called.

Version 0.1 of diagrams-pdf was using a function which has been
removed. Now the functionality is in renderDia as it should be.
So, renderDia is now use to run the tests.

With diagrams-pdf version 0.2, all tests are passing.
Corrected case problem for Pdf file names.
Merge pull request #1 from alpheccar/master

Added the pdf backend.