Add support for drip.

Drip allows for faster startup time by pre-starting JVMs and keeping a
pool of them around for use on-demand. More details here:
https://github.com/flatland/drip

Tested with this config:

* 'input { generator { count => 1 } } output { null { } }'

Without drip:

    % time bin/logstash -e '...'
    13.19s user 2.94s system 170% cpu 9.458 total

With drip (install drip, then set 'JAVACMD=drip' in env):

    % time JAVACMD=drip bin/logstash -e '...'
    0.03s user 0.46s system 6% cpu 7.008 total

2.5 seconds faster (27% faster)
Merge pull request #909 from jordansissel/support-drip

Add support for drip.
Merge git://github.com/logstash/logstash