amended DOI import plugin to check for and ignore DOIs already in the
repository.  Added a phrase to system.xml and 2 config values to
plugins.pl to allow control of the field holding the DOI and how it
deals with the "doi:" prefix on the value to account for local
differences.
Merge pull request #225 from Ainmhidh/3.3

amended DOI import plugin to ignore DOIs already present