Adding possibility to include metadata (Title, Author, Keywords, Subject, Creator, Producer) to merged PDF

Example code : 
```
pdf = open('existing.pdf', 'rb')
pdf2 = open('existing2.pdf', 'rb')
merger = PdfFileMerger()
merger.append(fileobj=pdf)
merger.append(fileobj=pdf2)
# infos must be something like this :
# {
#    u'/Title': u'My title',
#    u'/Author': u'My author',
#    u'/Creator': u'My Creator',
#    u'/Keywords': u'My Keywords',
#    u'/Subject': u'My Subjects',
#    [...]
# }
merger.add_infos(infos)
merger.write_pdf(open("merged_with_infos.pdf", 'wb'))
```