Add issue template

Author: mar10

.github/CONTRIBUTING.md

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+Thanks for contributing :-)
+
+In lieu of a own guideline, please have a look at the 
+[Fancytree Contributing Guidelines](https://github.com/mar10/fancytree/wiki/HowtoContribute#report-issues).

.github/ISSUE_TEMPLATE.md

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+Thanks for contributing :-)
+
+Please add info below, then remove all unneeded lines from this issue report.
+
+
+### Expected and actual behavior
+
+... (Maybe even a screenshot? Any hints on the browser's debug console?)
+
+
+### Steps to reproduce the problem
+
+  1. ...
+  2. ...
+
+Could you set up a jsFiddle (http://jsfiddle.net/mar10/6o3u8a88/) or
+Plunker (http://plnkr.co/edit/Bbcoqy?p=preview) ?
+
+
+### Environment
+
+  - Browser type and version:
+  - jQuery and jQuery UI versions:
+  - Fancytree version:    
+    enabled/affected extensions: