Allow user access project page but show a modal to sign up

It closes #785.

![screen shot 2014-10-18 at 9 44 59 am](https://cloud.githubusercontent.com/assets/230476/4690394/2c7cd73e-56e6-11e4-9918-0426d333e4bb.png)

Doesn't make more sense to create a `#partially_show?` since all other methods are already "fully"?