codec#on_event api change

Fixes #2140
Avoid static file corruption (small.png) on Windows

When using `logstash web` on Windows the Kibana Logo on the dashboard is broken. This is because it will return small.png with 479 instead of 480 bytes (removing a 0x0d).

Fixes #1796
fix doc inconsistency about the versions in the getting started guide

Fixes #2109
clean up some reference to logstash.net in benefit of the es site

Fixes #2109
fix the release notes ascii docs style

Fixes #2109
add a reference to the plugins page within the logstash getting started tutorial

Fixes #2109
Add /var/log/logstash as an empty directory.

Testing deb package

    % dpkg -c build/logstash_2.0.0.dev-1_all.deb | grep var/log/logstash
    drwxrwxr-x 0/0               0 2014-12-01 22:29 ./var/log/logstash/

Testing rpm package

    % rpm -qvlp build/logstash-2.0.0.dev-1.noarch.rpm| grep var/log/logstash
    drwxrwxr-x    2 root    root                        0 Dec  1 22:34 /var/log/logstash

Fixes #2146

Fixes #2159