Firefox fix. Hides and shows file save dialog.

Hides and shows file save dialog after flipping to add recipients.
Fix Travis
Merge pull request #74 from thinkpunk/master

Firefox fix. Hides and shows file save dialog.
Clarify nonce format
Fix #71
Revert "Merge pull request #74 from thinkpunk/master"

This reverts commit 27d7c4d577f841b8f596fa8701bb2c83d4b98620, reversing
changes made to e2de7e1ce300842159de17a30dc133e3a349d3d9.
Fix #72
Conservative file size estimate
Clarify documentation
Implement proper progress reporting.
Post blobs from worker.

Since Blobs are immutable, looks like Chrome won't copy data when
posting the message. This is noticable if you encrypt/decrypt a large
file: there's no longer a delay at the end of operation.