Avatar Upload (with Carrierwave)

I configured [Carrierwave](https://github.com/carrierwaveuploader/carrierwave) for easy file uploads.

I created a custom `:upload` input which makes use of [Jasny Bootstrap fileinput](http://jasny.github.io/bootstrap/javascript/#fileinput).

**The user now has the possibility to upload an avatar.** :smiley:

Select a new file:
![image](https://cloud.githubusercontent.com/assets/1814983/3479569/14f96248-0354-11e4-884f-76641f5b3e87.png)

File selected (allows to select another file):
![image](https://cloud.githubusercontent.com/assets/1814983/3479644/61ce26e8-0355-11e4-8565-1a9c6f7f1416.png)

Reset selection (back to no file, or to the already uploaded file, if available):
![image](https://cloud.githubusercontent.com/assets/1814983/3479591/6ef10404-0354-11e4-9d72-081f0c45e9a1.png)

Also has sort of a preview for non-image files:
![image](https://cloud.githubusercontent.com/assets/1814983/3479597/833406a0-0354-11e4-9fb4-d233bd4b8f0f.png)

Remove an existing image:
![image](https://cloud.githubusercontent.com/assets/1814983/3479627/1dcf6254-0355-11e4-94a0-6b89d50a7e8b.png)

The avatar is displayed in the mini hub:
![image](https://cloud.githubusercontent.com/assets/1814983/3479622/0b165028-0355-11e4-8ef0-429082928088.png)

**Note**

Form re-displays doesn't seem to keep the upload cache correctly! This seems to be a problem on Carrierwave's side, see https://github.com/carrierwaveuploader/carrierwave/issues/1414. There's a pending spec which shows the problem. Let's hope this will be fixed, soon.