Adjust credits
Adjust encrypt layout, align signature with button
Fix file decryption interface

Actually use mAssumeSymmetricEncryption and set it to false as
default. Don't close the file input stream, so we can actually
look for PGP encrypted data in the next step.
Remove icon set credits
Update README.md, icon set not used anymore
Merge pull request #473 from thi/master

Adjust credits, align signature in encrypt layout slightly, fix file decryption, remove icon set credits