Bugfix and code maintenance.

* Remove and sort usings in XmlSerializer.cs
* Add XML documentation to XmlSerializer members (yo dawg, we heard you
like XML...)
* Fix enum types not being properly deserialized if the enum fields
existed as an XML attribute.
* Enum parsing code was previously throwing an exception if the
requested field could not be found, now it will simply assign the
default value if the requested value cannot be found.