Modified "objectName" in "Group" to use native "H5Lget_name_by_idx".

"objectName" in "Group" now uses the native "H5Lget_name_by_idx" instead of the CommonFG wrapper method "getObjnameByIdx". Only but important difference: "objectName" will now not cause an exception if the index is not found but return an empty string.
Merge pull request #153 from balint42/rewrite_group-objectName

Modified "objectName" in "Group" to use native "H5Lget_name_by_idx".