MINOR: http: capture.req.method and capture.req.uri

MINOR: http: capture.req.method and capture.req.uri

Add 2 sample fetchs allowing to extract the method and the uri of an
HTTP request.

FIXME: the sample fetches parser can't add the LW_REQ requirement, at
the moment this flag is used automatically when you use sample fetches.

Note: also fixed the alphabetical order of other capture.req.* keywords
in the doc.