Move sanitise_header_text to to_string

sanitise_header_text has been move Frame::to_string as the headers
stored in the frame should be how the user would see them, they should
not reflect the STOMP protocol.
Add Trello board to README
Add content length header

Frames should have a content-length header according to the spec.
Calculate one when a Frame is created.
Add method to Message so users can get headers