Support GitLab projects

[alaxalves]

Expected Behavior

Falko could support projects from Gitlab too.

Current Behavior

Only Github API is supported, currently.

Checklist

• Issue has labels.
• Issue has a significant name.

We could use this gem:
https://github.com/narkoz/gitlab

[alaxalves]

Poderiamos usar a gem gitlab para facilitar a integração.

[alaxalves]

Depende da issue #228