Bump version numbers to 0.3

* Bump all version numbers to 0.3.
* Make gitlab-api-plugin use its own version for its gitlab-api-client.
Update POM files for hosting at jenkins-ci.org

* Add SCM information for all POM files.
* Add distribution management information for all POM files.
* Add developer IDs (at jenkins-ci.org) for all POM files.
* Add URL to the Jenkins wiki page for the POM files for the parent and
the Jenkins plugin.
Change Jenkins version to 1.554.1

Update POM file for gitlab-api-plugin to change the Jenkins version to
1.554.1.
Add description to POM files

* Add description to the POM files of gitlab-api-plugin-parent and
gitlab-api-plugin taken from the index.jelly file of the plugin.
* Add name and description to the POM file of gitlab-api-client
Change groupId to com.sonymobile.jenkins.plugins.gitlab

* Change the groupId to com.sonymobile.jenkins.plugins.gitlab and move
existing packages.
* Change URL for SCM since the parent project is not going to have
"parent" in its folder name.
* Fix package name in config.groovy.
* Fix bug getting the API client from GitLabConfiguration (possibly
introduced when switching to Jenkins 1.554.1).
Add README.md file for the parent module

Add a brief README.md file for the parent module and update README.md
for gitlab-api-plugin with a short usage section.
Merge branch 'release/0.3' into dev