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.

setup.py

@@ -8,7 +8,7 @@ from setuptools import setup
 
 requires = ['feedgenerator >= 1.9', 'jinja2 >= 2.7', 'pygments', 'docutils',
             'pytz >= 0a', 'blinker', 'unidecode', 'six >= 1.4',
-            'python-dateutil', 'livereload']
+            'python-dateutil']
 
 entry_points = {
     'console_scripts': [