Add "HTTP Model" to the wiki page "Module notes"

[fgeorges]

More context, from the minutes of the meeting in Prague in Feb. 2015:
https://lists.w3.org/Archives/Public/public-expath/2015Feb/0005.html

The group did not have time to discuss this point. Adam and Florent
expressed interest in working on a HTTP Model (containing an XML
structure definition, maybe expressed as a schema, and a set of
functions to access some parts of it).

The HTTP Client and Webapp modules would then be adapted to use this
common model, on both the client and server sides. Such a model could
also be used as a common format to store HTTP requests, e.g. in logs
or in test suites.