Add support for postfix_mailgraph

Adds support for postfix_mailgraph.
Requires mailgraph to be preinstalled, requires modification to mailgraph binary to use observiums polling time.
Detailed info: http://docs.observium.org/apps/#postfix-mailgraph

run Tests on Debian/Ubuntu/Centos

run Tests on Debian/Ubuntu/Centos with docker.

Include basic testing

0.4

add build badge to readme

Meta info improvements

0.3

provide desc

Fix ansible galaxy meta info

0.2

bump ansible version, fix platforms

Initial release

0.1

update galaxy info