Refactoring of init.php

- fetching is now done by a global function in fetch.php. This
  allows to use this functions in test/xpath.php for testing.
- besides of user configuration as explained in README.d json
  files from `mods` are now also loaded and used for fetching
  articles
- loading of json files is done only one time and not for each
  article to process
- test/xpath.php is now a script, which can be used to test a
  configuration from `mods`-directory

Added two mods for 4players and visions.de.

TODO: rewrite README.md

mods/visions.de.json

@@ -0,0 +1,11 @@
+{
+    "name": "VISIONS.de",
+    "author": "boxdot",
+    "feed": "http://rss.feedsportal.com/c/32350/f/443184/index.rss",
+    "match": "visions0Bde0C",
+    "config": {
+        "type": "xpath",
+        "xpath": "div[contains(@class, 'marginbt')]",
+        "force_charset": "utf-8"
+    }
+}