Support multiple versions of the plugin

[erik-edling]

Description

As can be read in #16, the plans are to finish the first version of the plugin for gerrit api version 2.14. When that is done, we should look into how to adapt this plugin to fit with the last version of the gerrit api taking to consideration that we should continue to support 2.14 as well.

Motivation

There are different users of the plugin that requires support for different version of gerrit api.

Exemplification

Master supports latest gerrit api version.
Branch 2.14 supports gerrit api 2.14
Any other branches could support other versions of gerrit if needed

Benefits

User is not restricted to a specific gerrit api version.

Possible Drawbacks

More versions to maintain.