make it possible for user to overwrite defaults

it's always unwise to set pre-defined, fixed flags. configuration should
be done in logstash.conf. making application run "out-of-box" and rc.d
script complex is a major PITA.

- add logstash_java_flags to set jvm flags, such as -Xmx${n}m
Merge pull request #51 from reallyenglish/tomoyukis

make it possible for user to overwrite defaults