Prefer conference proceedings over journal (newsletter) articles in the ACM Digital Library when both exist for the same paper.

Many papers originally published in ACM conferences are referenced in Special
Interest Group newsletters, which have journal article-type bibtex entries in
the DL.  For example, papers in PLDI will have bibtex entries for both the PLDI
proceedings and the SIGPLAN Notices newsletter, where info about recent
conference papers get reprinted.  The ACM Digital Library generally has
citations for both, and in unpredictable order.  Conference papers that are
later republished in "real" journals (e.g., TOPLAS) appear as completely
separate items in the ACM DL.  (Furthermore, unlike in many other science
fields, conferences are the main venue for substantial new computer science
research.  Journals are secondary, often offering a venue for expanded or
retrospective versions of original conference proceedings papers.)  This commit
fixes two issues with how the Zotero ACM DL translator currently handles items
that have both conference proceedings and SIG newsletter citations.

First, detectWeb() now prefers conference proceedings citations over journal
article citations if both appear for a given paper, instead of the reverse.
Since conference proceedings items that are also journal article items are just
newsletter copies and never "real" journal articles, it is fine (and correct) to
do this.

Second, scrape() previously took whatever entry appeared first in the bibtex
provided by the DL, but given the inconsistent order of proceedings
vs. newsletter items, it sometimes selected the the newsletter version instead
of the conference proceedings version.  This commit changes the translator so
that it tries to find a conference proceedings entry (and corresponding URL) if
it exists, and submits that to the bibtex importer rather than submitting the
entire bibtex listing as in the current method (from which the importer appears
to take whatever entry is first).  If no conference proceedings version exists,
it falls back on the current method.

Also: Update tests to match new/current correct behavior

Incorporated style tweaks per aurimasv.

And allowed conference proceedings titles and journal titles without spaces.
Merge pull request #641 from bpw/acm

ACM Digital Library: prefer conference proceedings papers where newsletter also exists