copyFile return based on type and file copyTo

- Changed the `Api::copyFile` method to only return a `File` instance if the API response type is `file` otherwise it returns a string.
- Added a convience method to the File class `copyTo` that expects a `$target`
Merge pull request #37 from adamgoose/master

Adding UUID Getter
use uuid instead of file_id
Merge pull request #36 from pmh1/master

copyFile return based on type