Update INSTALL.md

Corrected broken Link URL for Pre-compiled android apk
Merge pull request #614 from bob3434/docs

Update INSTALL.md
Update German translation
Merge pull request #615 from sfan5/patch-2

Update German translation
Only one instance of uTox can now be opened at a time on windows.
Increased maximum number of file transfers to 32.
Add optional Ringtone to uTox. Can be enabled in the settings page.
Merge branch 'Ringer' of https://github.com/bob3434/uTox

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Ringtone fixes.
Ringtone fixes: use looping source.
ringtones should now work with multiple callers.
Only generate the ringer sound once, clean up after.
Fixed utox_save.