Remove the manifest file

As of today (Nov 1, 2016) appcache is removed from the HTML5 spec.
Currently, the appcache isnt being used in Snap! and so there arent and changes in removing it.
We should look at other options for offline access for which there are many.

manifest.mf

@@ -1,13 +0,0 @@
-CACHE MANIFEST
-snap.html
-blocks.js
-byob.js
-gui.js
-lists.js
-morphic.js
-objects.js
-threads.js
-widgets.js
-store.js
-xml.js
-snap_logo_sm.png