BugFix for "#880 PubMed Import broken"

Found bug: PubMed/Medline PLAIN format recognized as RIS format.
Provided solution:
	- Modified RIS ImportFilter. Now RIS format is identified by looking for the "TY" tag (it is a mandatory tag in RIS format and it is not conflictual with other filters)
	- Added new ImportFilter "MedlinePlain". This import filter parses Medline files in plain format (not XML). The import filter has been produced referring to the official Medline documentation (http://www.nlm.nih.gov/bsd/mms/medlineelements.html)
	- Updated the net.sf.jabref.core plugin.xml file in order to include the newly added MedlinePlain import filter