Use HTML5 download instead of extension API

Benefits:
- Cross browser solution (however only for relatively new browsers)
- Doesn't need extra permission in Chrome

If the browser doesn't suppor the download attribute, then a new tab will
be opened with the exported data.

Other changes:
- Start the download only if the data is not empty (previously the
  download started anyway)
- Reorder code in vapi-client.js for Safari, so unnecessary code doesn't
  run on extension pages
Use a dedicated file for storing extension info

Chrome has getManifest(), Safari doesn't have anything, Firefox has an
asynchronous API...
So, instead of using extension APIs, store the common informations
(extension name, version, homepage url) in a file (vapi-appinfo.js), which
can be included when it's needed (its data will be available at vAPI.app.____).
The file's content is updated each time the extension is being built, so
it shouldn't be modified manually.