New webservice API

[Pierre-Yves Chibon]

> - change in the request mode to use
>   - POST is used to upload new dives
>   - GET is used to retrieve someone's dives
>   - the header 'Content-Type: text/xml' has to be used to retrieve the dives in
> 	xml format instead of the default json.

>From a discussion with Aurélien, this has just been changed to the 'Accept'
header. 'Content-Type' is meant to describe the format of the data that the
client sends to the server, while 'Accept' is meant to ask the server to return
the data in a specific format.

Pierre