Flush Graphite on close

When a large number of metrics needs to be reported, flushing
Graphite.writer in each call to send() greatly limits metric
reporting throughput.
Add Graphite UDP sender
create metrics-httpasyncclient module
instrument the NClientConnManager
Instrument the HttpAsyncClient using the builder
test
-
add module to parent pom definition
Change version to 3.1.0-SNAPSHOT
Merge branch 'master' of github.com:swissdave/metrics into swissdave-master
Fixes issues with GraphiteUDP

Use of default charset is disallowed, fix formatting
Merge pull request #594 from dropwizard/swissdave-master

Add Graphite UDP sender
Merge pull request #535 from maginatics/fix/flush

Flush Graphite on close
Merge pull request #570 from puniverse/add-metrics-httpasyncclient

Add metrics httpasyncclient
Update GraphiteRabbit PR
Merge pull request #554 from massimo-parente/master
Fix version hin metrics-httpasyncclient