statsd-ganglia: node 0.10+ compat; add custom filters feature

- Allow user to specify custom metric filters / preprocessors

  Not all metrics that are appropriate for Graphite are also appropriate for
  Ganglia. This patch adds an optional configuration option, "gangliaFilters",
  that the user may set to an array of module paths. Each referenced module is
  expected to export a "filter" function which takes a metric and either modifies
  it in some way, returns it unmodified, or returns a false value to exclude it
  from Ganglia reporting.

- Ensure compatibility with async sock.bind() in node 0.10+

  Per <http://nodejs.org/api/dgram.html#dgram_event_listening>: "the behavior
  of dgram.Socket#bind() has changed in v0.10 and is always asynchronous now."
  This means that setting the broadcast flag has to be deferred to a callback
  to ensure it does not execute before the socket is bound. Setting the flag in
  a 'listening' event handler is compatible with both old and new versions of
  node.

Change-Id: I978c86f4686f37facc44ca6f1af4b697ea9b83a2
Merge "statsd-ganglia: node 0.10+ compat; add custom filters feature" into production