According to HL7 spec, XPN field 9 is "Name Context" not "Contact"
Add support for MRG segments.
Unspecified fields should still set an attribute on the data type.

This avoids AttributeErrors when accessing field attributes that weren't set.
Unspecified attributes will be set to None.
Don't use name_contact in test since we have another pull request that renames that.
Raise AttributeError on invalid message attributes.

Current behavior is to infinitely recurse until RuntimeError: maximum recursion depth exceeded
Merge pull request #8 from rectalogic/invalid-attr

Raise AttributeError on invalid message attributes.
Merge pull request #5 from rectalogic/name

According to HL7 spec, XPN field 9 is "Name Context" not "Contact"
Merge pull request #6 from rectalogic/mrg

Add support for MRG segments.
Merge pull request #7 from rectalogic/attrs

Unspecified fields should still set an attribute on the data type.