Added the ability to use digest authentication instead of basic.

This changes introduces the ability to use "digest" authentication instead of "basic" authentication. This is more secure and also means the password is not saved or persisted in preference files or anywhere on the device.

Digest authentication can be added to (e.g.) Apache using the following config:
               AuthType Digest
                AuthName "home" <-- enter the "realm" here
                AuthDigestProvider file
                AuthUserFile /etc/apache2/auth/auth.conf <-- file created with htdigest
                Require valid-user

and use the htdigest program to generate a new digest.

I've been running this version of code on my n5 and my n7 and used both daily for the last two weeks and had no problems at all using this to access my newly secured ttrss instance.

Changes are only in the following files:

AndroidManifest.xml
res/layout/login_to_realm_with_digest_dialog.xml
res/values/strings.xml
src/org/fox/ttrss/ApiRequest.java
src/org/fox/ttrss/LoginToRealmWithDigestActivity.java
src/org/fox/ttrss/util/Utils.java

Changes to all the other files can be ignored - they were done because I think I've got newer versions of some of the libraries used so needed to make small corrections to get a valid compile. They are unrelated to this authentication change itself.