preventing_fund_to_compute_site_duplications

This is to prevent people assigning same compute site to the same fund more then once.
This is my last comment about formatting and hound (I promise). I'm writing it here because previous issue with hound comments was already closed.

Take a look at this movie http://vimeo.com/97329157 - for me it is quite convincing explanation why 80 chars per line is a good choise (more/less 18 minute of the movie).

```ruby
validates :compute_site_id,
           uniqueness: { 
             scope: :fund_id, 
             message: I18n.t('funds.unique_compute_site')
           }
```

E.g. when following format is used I'm able to see whole change using github without scrolling. IMHO even bed standard (for code format here) is better than having no format at all.

PS. At the beginning I reconfigured hound to prefer '' instead of "" for string without interpolation. What is more we can tweak hound configuration to improve it :smile: