pass opts on to dependent *_from_github methods

should be responsibility of get_contents_from_github to determine media type etc.
use remove_source_path opt when setting destination path

pass remove_source_path when copying a directory from github and the source path does not match the destination path

e.g. when copying

files/app/assets/javascripts -> app/assets/javascripts

set :remove_source_path => 'files/'