Bug fix: include volume number in citations of encyclopedias, etc.

Fixes Trac #61. This also fixes another issue: p. was being omitted
from bibliography entries for the item types this fixes. This is
contrary to the Norme, but was somewhat desireable insofar as some
items have column numbers instead of page numbers, and the style
cannot currently distinguish.
Don't use the locator label in contexts where the locator is undefined.

I was hoping this would get me "col." in the bibliography for things
that have a column number instead of a page number, but it doesn't
work that way. The Zotero DB doesn't even alow for non-page numbers.