Not case sensitive username

Username should be case insensitive.

Precondition:

1. Registered user with username 'testuser2014' and password 'testuser2014'
2. User is logged off

Steps to reproduce:

1. Open 'Sign in' page
2. Input in username field 'Testuser2014' ('TESTuser2014', ets.) and in password field 'testuser2014'.

AR: authorization failed and showed error message 'Invalid username or password'.
ER: authorization successful.