Move start page to be specified in <content> tag.

The default is still index.html and can be overridden in AppDelegate.m
if necessary. The <content src="PAGE" /> can be a relative path into www
or a full URL with scheme.
Merge branch 'master' into start_page
[ios] Remove %-escaping version of FileReader.readAsText()

https://issues.apache.org/jira/browse/CB-2168