add rtf and txt as supported file formats

[timar]

Fix for #10

[timar]

Fix for #16

[timar]

Analysing /home/travis/build/nextcloud/server/apps/richdocuments/lib/Controller/DocumentController.php
 1 errors
    line  225: OC_Helper - Static method of private class must not be called
Analysing /home/travis/build/nextcloud/server/apps/richdocuments/appinfo/app.php
 1 errors
    line   44: OC_Helper - Static method of private class must not be called
App is not compliant

These errors were not introduced by my patches, and I don't know how to fix them, if it is possible at all, and this pull request has been blocked for two months because of this. It is quite discouraging.

@LukasReschke can you please move this forward?

[LukasReschke]

I'm not entirely happy about this because for TXT we already do have a standard viewer for example but this seems fair enough. Might warrant in the server to offer some "With what app do you want to open it" in the future.