Implement Roles list on InformationSource API

Fix #144 
These `from_foo()` methods should be `VocabString.from_foo(...)` rather than `InformationSourceRole.from_foo()` since we don't actually know if the value being parsed in is using the default vocabulary or not. `VocabString` peeks at the input attributes like `xsi:type` and determines which `VocabString` implementation to create.