Move online documentation to readthedocs.io. Issue #17.

Author: RKrahl

README.rst

@@ -66,7 +66,7 @@ The directory doc/html in the source distribution contains the
 documentation in HTML form generated by Sphinx.  The starting point is
 doc/html/index.html.  The documentation can also be found online at
 
-    http://pythonhosted.org/pytest-dependency/
+    https://pytest-dependency.readthedocs.io/
 
 The example test modules used in the documentation can be found in
 doc/examples in the source distribution.