replaced the term "paste" with the more generic "document"

Some of the references to "paste" in code or comments got changed as well, but to clarify the intended usage of the terms:

- A PrivateBin document can consist of a paste text (key "paste" in the encrypted payload) and one or several attachments and discussion entries.
- Internally the root document is called a "Paste" and each discussion entry is called a "Discussion".
- When referring to a whole document with one paste and optional discussion(s), we call it just "document".
- When talking about a particular JSON payload type in the internal logic, i.e. during storage or transmission, we call them a paste or discussion to distinguish which type we refer to.

closes #397

lib/Request.php

@@ -75,7 +75,7 @@ class Request
     private $_isJsonApi = false;
 
     /**
-     * Return the paste ID of the current paste.
+     * Return the paste ID of the current document.
      *
      * @access private
      * @return string