Previously, a few people have been confused by an empty list when they first
open F-Droid (e.g. if they are not connected to the internet, and repos didn't
update). This provides some feedback so that there is never a blank screen.
Fixes Issue #34.
When adding repositories using the Manage Repos activity, firstly look
for an /index.jar appended on the URL provided by the user. If that
doesn't work (HTTP status code other than 200) then it will try
/fdroid/repo/index.jar, then /repo/index.jar. If it can't establish a
connection to the server, or if none of the above attempts results
in a 200, then the path provided by the user is kept (even though we
have a hunch it might be wrong).
This is to cover for the case where people arn't connected to the net.
Another way to deal with no internet connectivity is provided by a
"Skip" button on the dialog while searching for the index.jar.
The searching for index.jar is done by doing a HTTP HEAD request, so
the entire jar needn't be downloaded.
Finally, to make this happen in a clean sort of way, I refactored the
ManageReposActivity a little bit to encapsulate all of the add repo
dialog handling into a subclass. This way, the outer class doesn't
need to know things like: Is the dialog showing, what state is it in,
is the background task to search for index.jar files running, how and
when to cancel that task, etc.
