Add setting to disable using default image for the people page.

#4814 

After addressing the design in https://github.com/openscholar/openscholar/issues/4814#issuecomment-32818088 we now have in the os_profiles app settings in ``/cp/build/features/os_profiles``:
![selection_133](https://f.cloud.github.com/assets/4497748/1963889/3dcfeaec-82a4-11e3-9f23-0b17b3d1c998.png)

When an image is uplaoded:
![selection_134](https://f.cloud.github.com/assets/4497748/1963892/463daf16-82a4-11e3-899f-0ced21cad544.png)