[#4412] - Allow to specify template when exporting to PDF (patch included)

I often found myself wanting to print my generated reports onto a PDF containing our business letterhead.
This patch adds a JRTemplatePdfExporter which takes a template parameter pointing to an existing PDF file.
The report is then merged with this template in the following fashion:

a) if the template has only one page, this page will be printed behind every page of the report
b) if the template has two pages, the first page will be used for the first page of the report while the second page will be used for the remaining report pages
c) if the template has three pages, the third page will be be used on the last page of the report