json: add event_type key

This patch adds an event_type key to the generated events. Current
value is one of "dns", "alert, "file", "tls", "http", "drop". It is
then easy to differentiate in log analysis tools the events based on
source inside Suricata.
json file: separate http params

This patch separates http keys from file to have a different value
list:

{
 "time":"01\/31\/2014-12:04:52.837245","event_type":"file","src_ip":"5.3.1.1","src_port":80,"dest_ip":"1.8.1.9","dest_port":9539,"proto":"TCP",
 "http":{"url":"/foo/","hostname":"bar.com","http_refer":"http:\/\/bar.org","http_user_agent":"Mozilla\/5.0"},
 "file":{"filename":"bar","magic":"unknown","state":"CLOSED","stored":false,"size":21}
}

One interest of this modification is that it is possible to use the
same key as the one used in http events. Thus correlating both type
of events is trivial. On code side, this will permit to factorize
the code by simply asking the underlying protocol to output its
info in a json object.

Second interest is that adding file extraction for a new protocol
will result in only changing the protocol specific json list.