Text.CSL.Output.Plain: Code cleanup for clarity and readibility.
Text.CSL.Output:  Renamed <> to <^>.

<> is standardly used for Monoid append.
Text.CSL.Style:  Rename betterThen -> betterThan.
Text.CSL.Output.Pandoc:  Added Span to traversal routines.

This fixes a bug in which periods would sometimes not be
inserted at the end of a title (if it ended with a nocase span).