Merge pull request #4 from LumenTeun/master

Added an F-Droid link to Sipdroid. Thanks!
fixed design issue with https
fixed design issue with https
major rewrite by LumenTeun
index/credits fix, minor cleanup
Added a bigger version of the F-Droid logo and changed the source for the F-Droid logo at the bottom.
Fixed the Min. Android version numbers.

Fixed the Min. Android version numbers according to these sources:
https://wiki.mozilla.org/Mobile/Platforms/Android#System_Requirements
https://play.google.com/store/apps/details?id=com.csipsimple
https://play.google.com/store/apps/details?id=org.videolan.vlc.betav7neon
https://play.google.com/store/apps/details?id=org.openintents.filemanager
https://play.google.com/store/apps/details?id=net.osmand
https://play.google.com/store/apps/details?id=org.openintents.flashlight