DNSimple get_domain also accepts domain name

As the real DNSimple behaviour, get_domain also accept a string argument.

```
[1] pry(main)> require 'fog/dnsimple' 
[2] pry(main)> dnsimple = Fog::DNS.new(:provider => "DNSimple", :dnsimple_email => 'test@example.com', :dnsimple_password => 'mypassword')
[3] pry(main)> dnsimple.zones.get('wadus.me')
=>   <Fog::DNS::DNSimple::Zone
    id=1969,
    domain="wadus.me",
    created_at="2011-02-28T09:49:56Z",
    updated_at="2014-01-10T15:16:32Z"
  >
[4] pry(main)> dnsimple.zones.get 1969
=>   <Fog::DNS::DNSimple::Zone
    id=1969,
    domain="wadus.me",
    created_at="2011-02-28T09:49:56Z",
    updated_at="2014-01-10T15:16:32Z"
  >
```