added foreignKey method to AnnotationDB and __getattr__ for AnnotationSeq and AnnotationSlice to forward attributes from the corresponding slice object
added foreignKey method to AnnotationDB and __getattr__ for AnnotationSeq
and AnnotationSlice to forward attributes from the corresponding slice object

ctb/pygr,theoryno3/pygr,RitwikGupta/pygr,ctb/pygr,ctb/pygr,cjlee112/pygr,GenDataPro/pygr,cjlee112/pygr,GenDataPro/pygr,cjlee112/pygr,cjlee112/pygr,theoryno3/pygr,ctb/pygr,RitwikGupta/pygr,GenDataPro/pygr,ctb/pygr,theoryno3/pygr,RitwikGupta/pygr