adding the ability to specify a url as well as binary data

per https://jira-projects.socialcast.com/browse/APA-25

Use `RestClient.get` to simply convert a URL into a binary string

``` ruby
1.9.3-p484 :015 > binary = RestClient.get 'http://www.vmware.com/files/images/framework/logo-b.png'; nil
 => nil
1.9.3-p484 :016 > tempfile = Tempfile.new('myimage'); nil
 => nil
1.9.3-p484 :017 > tempfile.write(binary)
 => 2397
1.9.3-p484 :018 > tempfile.path
 => "/var/folders/w8/krgx2sb51qs0vngqx5_0nvl00000gn/T/myimage20140402-70030-13cvqjz"
```