Thomasharte/login activity

As implemented, the login/signup activity is implemented:

* as a tabbed view pager, switching between login and sign up screens;
• with the login screen offering email address and password boxes, plus a login button and an 'I forgot my password' link; and
• with the sign-up screen offering two e-mail address boxes (one being 'confirm') plus a password box and a 'sign up' button.

The login button on the first screen will find the existing local user with that email address if one exists, and will otherwise create that user. It will then supply it as the BusinessFormActivity.USER when firing off an intent to start the BusinessFormActivity.

I'll autonomously accept this pull request later today if no negative feedback is forthcoming; I just thought this was a more comfortable place to discuss the code stuff and to solicit feedback.