PDF: Add support for custom page size

Clone the behavior of PostScript: If the property PAGE_SIZE is set to
A4, LETTER etc, then use that. But if it is set to the value
CUSTOM_PAGE_SIZE, evaluate the dimension value of the property with key
CUSTOM_PAGE_SIZE.
Merge pull request #27 from gcauchis/master

[EMF] Little improve of freehep-graphicsio-emf for font quality and image ratio.
Merge pull request #25 from hfs/pdf-custom-page-size

PDF: Add support for custom page size
Updated plugins
[maven-release-plugin] prepare release freehep-vectorgraphics-2.4