Added Walkable instances for Citation.

This address pandoc #1269 -- previously `walk` did not enter
into Citation prefixes and suffixes.
Bump version to 1.12.3.3.
Added travis build script.
Travis: use ghc 7.8.3.
Added NFData instances for deepseq
Version bump to 1.12.3.4
Merge pull request #11 from mpickering/master

Added NFData instances for use with deepseq
Version bump to 1.12.4
Merge 1.12.4 from jgm

Conflicts:
	Text/Pandoc/Walk.hs