#18 support for merging odd-even pages from separate documents

[Berteh]

The patch below allows to shuffle the pages of a new document into the pages of the current document. Please add it to your main code for next release.

Say you scanned odd pages in doc A.pdf and even pages in doc B.pdf, simply open A in PDF-Shuffler, and add B with the option "shuffle into" checked, resulting document will have A1.B1.A2.B2.A3.B3...

Documents A and B need not be of same length.

Tested in Ubuntu 14.04, expected to work everywhere.