Updated texts for presentation module.
Allow downloading of presentations.
Revoke temporary URLs when closing.
Cleanup temporary files on "stop".
Fixed rendering of odf documents.
Added comment about revoking URL.
Merge pull request #80 from fancycode/download_presentations

Allow downloading of presentations.
Cleanup "blob:" url when file is stopped.
Merge pull request #81 from fancycode/cleanup_files

Cleanup files