Adding exif node tests

@dceejay, @knolleary Here's finally a "Good Path test" for the exif node. However do notice that:

1) It doesn't run automatically
2) The I ran it by calling grunt installed on the web nodes project:

<code>istanbul cover ../../../../node-red-web-nodes/node_modules/.bin/grunt  --report lcovonly && istanbul report html</code>

3) We need to decide if/when we're developing a test runner for these nodes