Ignore mkdocs site directory

[mkdocs](http://www.mkdocs.org/) is rising as an alternative to Sphinx for project's documentation. The default build command puts the generated documentation in a `site` directory at the root of the project which should be ignored.

Python.gitignore

@@ -92,3 +92,6 @@ ENV/
 
 # Rope project settings
 .ropeproject
+
+# mkdocs documentation
+site/