109 send fb notificacions

This PR allow group member to share "join" link using FB send button.
This PR add a public page with the required metatags and actions:
User logged in:
![fwc_join_1](https://cloud.githubusercontent.com/assets/357127/3168144/c8425d2e-eb7a-11e3-80a3-df97d81da8a0.png)
User not logged in:
![fwc_join_2](https://cloud.githubusercontent.com/assets/357127/3168146/d6342fe8-eb7a-11e3-8ec1-3e6842c1a470.png)

Group view:
![fwc_fb](https://cloud.githubusercontent.com/assets/357127/3168150/de89aaf6-eb7a-11e3-8df1-73eb142e16e6.png)