Enh/Fix: Better collectd module.

Enh:
 + Separated collectd parser code :
    imported collectd_parser.py from https://github.com/gst/python-collectd-parser/compare/misc_enhancements_v1?expand=1
 + Package importable as shinken-1.4 and 2.x plugin.
 + Purge elements which don't update anymore.
 + Optionnaly use a dedicated thread for reading collectd data.

Fix:
 + Good float encoding ("%f" instead of str())
 + Don't send too early perf data (wait 2*interval time before first send)
 + Need bigger buffer.