webstaff: add egJsonExporter directive

webstaff: add egJsonExporter directive

This directive is used to allow a piece of JSON
to be saved to the user's filesystem.  For example:

<span eg-json-exporter container="foo" default-file-name="example.json">Export</span>

specifies a button that, when click, allows the user to save
the contents of the scope variable foo.  The dialog that
appears will use "example.json" as the default filename.

This also adds a new dependency, the MIT-licensed
 angular-file-saver service written by Philipp Alferov:

https://alferov.github.io/angular-file-saver/

Signed-off-by: Galen Charlton <gmc@esilibrary.com>