Security update: correct misuse of `add_query_arg()` as identified by Sucuri: https://blog.sucuri.net/2015/04/security-advisory-xss-vulnerability-affecting-multiple-wordpress-plugins.html.

Security Update
* Correct security vulnerability allowing both private and password protected posts from being accessed through the print page
* Creates is_protected() method to determine if the print page should be visible to the current user
* Remove print_url links from the content when the current user does not have the necessary capabilities to view the print page
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* Add additional rewrite rules for situations where verbose page rules are required. Fixes #1.
* Disable canonical redirect when print template is requested. Also fixes #1.
* Update is_print() method to use WordPress API.
* Correct translation string implementation.
* Update code to better conform to WordPress Coding Standards.