drive-by fix of check for system collections in sharding #740
make new sharded collections use a primaryKey #740

Added a check for a nonexistent collection during shardCollection, and if
the collection doesn't exist yet, use the primaryKey feature of create to
make a primaryKey that contains the shard key.  This makes it so we can
have a clustering index on the shard key without paying twice for space.

closes #740
Merge remote-tracking branch 'origin/sharding-with-pk' into custompk