Add citation export tool

[ruthlane]

from Arlen:
I guess the thing that would be very nice for our users would be if the citations could be easily imported into bibliographic software. I guess that means being able to generate files in some of these formats:

Zotero RDF.
MODS.
BibTeX.
RIS.
Refer/BibIX.
Unqualified Dublin Core RDF.

I think BibTeX and RIS are the most common. As a Zotero and Endnote user, I know that 'book section' citations are usually the most cumbersome to generate, so generations of scholars will love us if we can make it easy to download a citation file, all ready to go.