Removing unnecessary fields from ProviderItem.

I'm targeting to refactor the whole ProviderItem class because I've not
used the Provider class so far and I should not duplicate information
from "Provider" into "ProviderItem".
Preseeded providers only have provider.json url

I've refactored everything to be able to remove all but provider.json
url at urls files.
Simplified the methods to add a new provider.

Adding a new provider is the same that selecting a new one, using the same methods and following the same workflow.
Successfully rebased onto progress-bars.
Provider setup: One method downloads everything.

This method uses another 3 to download each file.

Next step: broadcast progress and remove unnecessary methods.
A bit of cleaning.
ProviderAPI broadcasts progress to CW.

Setting up a provider makes the progressbar move according to the number of files downloaded (1/3, 2/3, 3/3).

Next step: recover from download errors individually. That means that if a download fails but the others went OK, the user will be prompted to retry that individual failing download.
CA cert saved correctly in SharedPreferences.

I/O error at login phase fixed.
Successfully rebased onto 0.2.2
Download results stored in static variables.

ProviderAPI has now 3 different static variables, each one for the 3 files that have to be downloaded to set up a provider.
Save my work until now
Retry button on failed downloads.

Next step: remove the provider from the list (big security hole, since all providers present on the list are assumed to be secure).
Not added item is removed after canceling dialog.
Cleaner error message.
JavaDoc for pickErrorMessage
Retry button prompts new provider dialog.

The user can change the new provider's data entered before failing.
New provider dialog remembers previous data.

It also (supposedly, I didn't test because Bitmask is working ok from our client point of view) distinguishes between cancelling a new custom provider and cancelling a preseeded provider.
Rebased onto current develop.
Rebased onto 0.2.2
Rebased onto 0.2.3.