Issue 1: File download in Safari

Load Blob.js in Safari, and add more descriptive message for users.

Currently it seems, in some versions of Safari, the only workaround is for
users to press Save As after the .ics contents open in a new window (see
https://github.com/eligrey/FileSaver.js/issues/12).

Change href='#' to javascript void to remove "Save link as" option on
right click (which would save the unwanted html file).

Add success/instructions alert on all successful runs.

Change version number to yymmdd format.
Use callbacks on $.getScript()

Only run the rest of the script after filesaver.js (and maybe blob.js) are
loaded. This fixes an issue of sometimes having to click twice before the
bookmarklet works.