Super-F-Droid Mergeset 1 for test releases

This merge request implements 4 different installers:

• SystemPermissionInstaller: uses internal Android methods with reflection to install apps. Is only used when F-Droid is installed as a system-app (for example by shipping it with Replicant)
• RootInstaller: Uses root access to install apps. Must be enabled via preference
• DefaultInstaller: Current installer based on Intent actions for Android < 4
• DefaultInstallerSdk14: Similar to DefaultInstaller, but uses explicit Intent actions for install/delete introduced in android 4.

Some other notes:

• new dependency as git submodule: libsuperuser (developed by chainfire and extended by me)
• fix https://gitlab.com/fdroid/fdroidclient/issues/4

Roadmap: This merge request is intended for -test releases to verify that the installers are working. Before a final release I will implement permission diff dialogs for RootInstaller and SystemPermissionInstaller in the next mergeset 2.

Future mergeset 3 should contain code to do multi-updates and/or unattended updates in the background.

See https://f-droid.org/wiki/page/Super-F-Droid to read my (messy) wiki page.