removes the patterns from patterns dir

The core patterns are now provided by the logstash-patterns-core gem.
This empty folder can be used for custom patterns.

Fixes #2228
Refactor doc generation

- Use a generic task for installing all plugins
- Use refactored bundle install task

Fixes #2258
Add comment for locale in date filter.

For non-english environment the date filter don't parse correctly the month name. See : http://stackoverflow.com/questions/26653490/logstash-date-filter-failed-parsing

Fixes #1987