Fixing and revamping ACM's Digital Library

Fixing and revamping ACM's Digital Library (a.k.a. ACM Portal) online search
which became necessary due to a recent redesign of the homepage. Less HTTP
requests are necessary now as the bibliography data for multiple items can be
fetched in one request. Data returned by ACM is in JSON format.

This is a forward-port of commit 925a20cd from branch 'kbibtex/0.10'.