auth navbar - added bare mode and PEP8 tweaks

auth navbar more flexible. Now it's simple to obtain a customized navbar like this (see the example in the commited code):  

![_navbar1](https://f.cloud.github.com/assets/3496662/1052229/c16c4158-10d9-11e3-8ab4-92e91676c16a.jpg)
![_navbar2](https://f.cloud.github.com/assets/3496662/1052228/c15dcf92-10d9-11e3-8f8d-2f29b19f4169.jpg)

without to create several navbar modes. Moreover the navbar function is more compliant with PEP8 rules.