FIX: set and arbitrary page size with CSS rule

When using the CSS rule to set arbitrary page size (i.e. not in
xhtml2pdf.default.PML_PAGESIZES), an error is raised.
pisaCSSBuilder requires a tuple of values to set width and height of a
page in his  self.c.pageSize
Merge pull request #79 from Kadmillos/master

Fix for a specific page size (different from standard ones)