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
updates docs to fix #2240

Fixes #2243
Small change so apt-transport-https package is not required on Debian. Yum supports https by default.

Fixes #2243