Update livereload Invoke task and add docs

Removes the `livereload` dependency from `setup.py`.

Updates the `invoke livereload` task by moving the `livereload` import
into the task function since it is now an optional dependency.

Updates the Invoke section of the documentaion with instructions on
using the `livereload` Invoke task.

docs/publish.rst

@@ -146,6 +146,12 @@ http://localhost:8000/::
 
     invoke serve
 
+To serve the generated site with automatic browser reloading every time a
+change is detected, first ``pip install livereload``, then use the
+following command::
+
+    invoke livereload
+
 If during the ``pelican-quickstart`` process you answered "yes" when asked
 whether you want to upload your site via SSH, you can use the following command
 to publish your site via rsync over SSH::