Refactor `Base_Object::get_file()` to use cURL API

* Support for URLs via `file_get_contents()` may not be supported by some web hosts
* Fix #82
Merge branch 'iss-url-fopen' into develop
Client: Add global public stylesheet
Client: Hide DOM elements when viewer displayed

* Flash objects, etc.
Merge branch 'opt-element-visibility' into develop
Update readme (2.1.3)
Update version (2.1.3)
Merge branch 'release-2.1.3'