Fixed unnecessary extra tab from opening on Firefox

Removed reference to target_view = view.open() to prevent the new tab
from opening. Instead catching this case within the fs_error function
with an else statement, so if no target_view: window.open() and pass
the target of "_blank" to allow dataURL to open in new tab rather than
steal the current window.
Updated to reflect Firefox extra tab fix on FileSaver.js

Reference #e2dd8c4b22
Merge pull request #33 from missing/master

Fix for unneeded extra tab that opens on Firefox.