Add extension points

[tsmock]

This is necessary for #1 and #16.

As far as I can tell, this does not yet work.
EDIT: Looks like the directories are not as expected

[flathubbot]

Started test build 140405

[flathubbot]

Build 140405 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/123445/org.freedesktop.Sdk.Extension.openjdk.flatpakref

[flathubbot]

Started test build 140410

[flathubbot]

Started test build 140413

[flathubbot]

Build 140413 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/123453/org.freedesktop.Sdk.Extension.openjdk.flatpakref

[flathubbot]

Build 140410 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/123450/org.freedesktop.Sdk.Extension.openjdk.flatpakref

[flathubbot]

Started test build 140430

[flathubbot]

Build 140430 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/123470/org.freedesktop.Sdk.Extension.openjdk.flatpakref

[flathubbot]

Started test build 140625

[flathubbot]

Build 140625 failed