WIP stream payload in lua

Allow a script to set the 'stream' buffer type. This will add the
script to the PMATCH list.

Example script:
alert tcp any any -> any any (content:"html"; lua:stream.lua; sid:1;)

    function init (args)
        local needs = {}
        needs["stream"] = tostring(true)
        return needs
    end

    -- return match via table
    function match(args)
        local result = {}

        b = tostring(args["stream"])
        o = tostring(args["offset"])

        bo = string.sub(b, o);
        print (bo)

        return result
    end

    return 0

Prscript:
- PR build: https://buildbot.openinfosecfoundation.org/builders/inliniac/builds/23
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/inliniac-pcap/builds/23