Add copy reference endpoints

[hmnhf]

• https://www.dropbox.com/developers/documentation/http/documentation#files-copy_reference-get
• https://www.dropbox.com/developers/documentation/http/documentation#files-copy_reference-save

[hmnhf]

@Jesus Hi, should I fix the style violations pointed out by Hound? I tried to stick to the style of existing code and Hound's style guide seems to be different.

[Jesus]

Hi, many thanks for your PR.

Please, ignore Hound's comments. I'll review and merge your changes this week.

[Jesus]

Your commit was very good, I only decided to change the class names so they match the names in Dropbox's documentation.

I will release a new version of the gem so your contribution this is available for everyone.

Many thanks!