[oslc-core] Addition of Common Properties: dcterms:abstract and dcterms:relation

[Steve K Speicher]

These common properties have emerged in CM and QM specifications, 
therefore I have added to Common Properties.

There may be cases where dcterms:title may have been used when 
dcterms:abstract may be more appropriate.
Also, dcterms:relation is a general purpose property that can be used to 
identified a generally related resources.

[1] http://open-services.net/bin/view/Main/OSLCCoreSpecAppendixADRAFT

Thanks,
Steve Speicher | IBM Rational Software | (919) 254-0645