Rewrite hl7 messages

[tomunger]

I made changes to accomplish two things.

First, Segment.assign_field() correctly re-writes values. The caller can redefine the structure of the hl7 message.

Second, a new function in Message and Segment, write_field(), first escapes the field and then calls assign_field(). This was my solution to provide write-with-escape in a backward compatible way.