Warn if we can't load credentials from ~/.fog instead of raising

This will allow us to use the local (Fusion) driver:

```ruby
require 'fog'
require 'fog-octocloud'

octoc = Fog::Compute.new( :provider => 'octocloud' )
```

See #10
Merge pull request #11 from rubiojr/issue-10

Warn if we can't load credentials from ~/.fog instead of raising