Base frame header API on the net/textproto API

[jjeffery]

Following on from issue #3, where it was suggested to put the header in a map rather than an array. Although a map implementation was considered originally, it was decided to use an array in order to (a) have some certainty about the order of header entries, and (b) handle repeated header entries in a deterministic manner.

I think a better implementation would be to have an API similar to the MIMEHeader type in the excellent net/textproto package (see http://golang.org/src/pkg/net/textproto/), but this would be a breaking change. I would probably create a StompHeader type based on MIMEHeader, but with the STOMP header rules (case-sensitive, no spaces, etc).

I am not sure how many people are actually using this library, and so I have no idea how much a breaking change would irritate people. Comments welcome.