Refactor netflow codec.

Netflow field definitions are now extracted to a bundled YAML file and also
introduces a new config directive to specify an additional YAML file that
can be used to augment the built-in list. This means adding new Netflow field
types doesn't require a rebuild of Logstash.

TODO Work out how to get the YAML file added to the jar.
Add example YAML to documentation
Pull Netflow YAML file into the jar.
Add syslog input test for priority bug
Fix priority detection issue with syslog input
Better error handling for loggly output
Make sure the i18n gem is v0.6.6 or newer
because lib/logstash/runner.rb uses `I18n.enforce_available_locales=`
which isn't available until v0.6.6.
Fix `bin\logstash.bat deps` on Windows and add coaching for common bootstrapping problems.
Merge pull request #808 from bodgit/netflow

Break out Netflow v9 field definitions into YAML file
Merge pull request #986 from jbussdieker/syslog_priority_fix

Syslog priority fix
Merge pull request #987 from jbussdieker/loggly_error_fix

Better error handling for loggly output
Merge pull request #989 from GregMefford/patch-2

Fix `bin\logstash.bat deps` on Windows
Merge pull request #988 from GregMefford/patch-1

Make sure the i18n gem is v0.6.6 or newer
Merge git://github.com/logstash/logstash