NullPointer solved.

ConfigHelper refactored methods returning nulls instead of empty objects
broke LeapHttpClient getInstance implementation, because it checked the
emptiness of the object and not if it was null.
Merge branch 'MeanderingCode-feature/authGui' into feature/authGui

This is the pull request #3 from GitHub.
OpenVPN certificate is downloaded from API_BASE, using api_uri field
from provider.json.

This fixes bug #2780
Removed unused methods and variables.

Variables from LeapSRPSession were there because I used it while testing
srp calculations, comparing that strings with the ones from javascript.

Unused method from ProviderAPI was there because I foresee I'll have to
implement it in the future, but I've removed it since it's already in
the history.

This fixes #2781.
We are not using SD storage anymore.

We were using it to save certificate and provider.json files from chosen
provider, so that exporting them was easily done.

We don't need that files, because if we wanted to export that files we
would be able to recover them easily and updated.

This fixes #2783
Removed some unnecessary libraries.

I've also removed some unnecessary comments from LeapHttpClient.
bitmask json files are downloaded.

There was a problem on the assets file "bitmask.url". It had an error in
the eip-service url.

We should use this file only for main url, and proceed as if it were a
new provider but with a preseeded main url.
Login is working OK.

I removed a library that was not directly used by my code, but for one
of the libraries that I needed.
Bypasses self signed certificates.

It's working against cdev.bitmask.net and bitmask.net. Look at #2840 for
further explanation about self signed certificates.

I've also removed some file dependant configuration (when a provider was
custom, ConfigurationWizard still tried to read from file a
provider.json that now I store in memory via ProviderItem class).