add possibility to search for posts by DOI, ISBN and URL via the API

[Former user]

Reported by dbe: Currently, it is only possible to retrieve posts by tag, users or groups. This should be extended to retrieve posts by - DOI - ISBN and - URL

We will have to see whether we incorporate this into a fulltext search feature or if we do a separate solution for these fields.

[Robert Jäschke]

Can be done via "system-tags" after migration to Lucene

[Robert Jäschke]

Hhmm, don't see this as important now.

[Daniel Zoller]

can now be done by elasticsearch

e.g. http://www.bibsonomy.org/search/isbn%3A3642380557