Better signin UX for users. Refs #251
Refactor some code. Refs #251
Refactor code. Closes #251

This final commit includes the unit tests for the problems when signing in into PyBossa.

The new system gives the user a hint about which method he could used in the past, when creating an account, so he can recover the password or use the right method to sign in again.
Add the Twitter and Facebook IDs for testing. Refs #251
Merge pull request #397 from PyBossa/issue-251-user-signin

Issue 251 user signin