This change adds server support for the GetAttributes operation.
The user can specify an object ID and an attribute list to get
the values of the attributes listed from the specified object.
The user can also omit either argument; the server will default
to using the ID placeholder and all viable attributes respectively.
Only a subset of the standard attributes are supported right now.
New tests have been added to cover the new feature.
Closes#185Closes#211Closes#218