refs #594 Make the IndexSpec::missingField() implementation IndexType specific, and use missingField() to properly identify missing fields in CheckShardingIndex::run().

Conflicts:
	src/mongo/db/hashindex.cpp
	src/mongo/db/hashindex.h
	src/mongo/db/indexkey.cpp
	src/mongo/db/indexkey.h
	src/mongo/dbtests/namespacetests.cpp
	src/mongo/s/d_split.cpp