Replace 404 with HttpURLConnection.HTTP_NOT_FOUND

(cherry picked from commit 8ea9951)
Tests: test GET/DELETE on non existing repo

`GET` and `DELETE` should return `404` error.

Closes #28.

(cherry picked from commit 4270665)
Add a missing trace to `removeContainer`

(cherry picked from commit 3cd0e89)
Docs: make the welcome page more obvious

One of the concern we have with our documentation is that it's hard for users to understand which plugin version they should use with a given elasticsearch version.

The change will consist of:

* have a clean and simple compatibility matrix on master branch
* remove list of versions in other branches

Closes #29.

(cherry picked from commit 845e525)