added GENERIC_LOGIN_REQUIRED setting for a full authenticated graphite site

I've added a new setting to enable a full authenticated site, related to issue  https://github.com/graphite-project/graphite-web/issues/681

Is only tested with internal django user handler, not tested yet with remote or ldap authentication.

Also I've added a new look&feel for the login form: 

The old login form was:

![image](https://cloud.githubusercontent.com/assets/5883405/2697218/b55c2714-c3eb-11e3-970d-79c43dda2313.png)

And the new one:

![image](https://cloud.githubusercontent.com/assets/5883405/2697226/d0b4359c-c3eb-11e3-8809-e1b6eb003254.png)