Validate that the start_url member in the manifest is a full URL

[f2bo]

Even though a relative URL is a valid start_url according to the W3C spec, ManifoldCordova requires a full URL (because the plugin cannot determine the manifest's origin once it has been downloaded).

The plugin should validate this member and issue a suitable error message if it finds a relative URL. There should be also an initial check that the start_url member actually exists in the manifest, and if not show an appropriate error message.

[f2bo]

Added the necessary validation - see #21.