Bump version to 0.3

* Bump the version number in the POM file.
* Bump version number of gitlab-api dependency to 0.3.
Change groupId to com.sonymobile.jenkins.plugins.gitlab

Change the groupId to com.sonymobile.jenkins.plugins.gitlab and move
existing packages.
Update groupId for dependency to gitlab-api

Update the groupId for the dependency to gitlab-api to
com.sonymobile.jenkins.plugins.gitlab.
Update POM files for hosting at jenkins-ci.org

* Add SCM information for the POM file.
* Add distribution management information for the POM file.
* Add developer IDs (at jenkins-ci.org) for the POM file.
* Add URL to the Jenkins wiki page for the POM file.
Add README.md

Add a brief README.md file for the plugin.
Merge branch 'release/0.3' into dev