protect PluginLoader._extra_dirs from appending None

PluginLoader.add_directory() can receive None from, for example, 
Inventory.add_directory(self.basedir()) if host_list is a custom list.
None has no reasonable interpretation other than ignore it.
Merge pull request #1900 from bostrick/devel

protect PluginLoader._extra_dirs from appending None
service module 'state' parameter comments should be processed by RST
Merge pull request #1905 from bct/service-doc-formatting

Minor doc fix: service module 'state' parameter comments should be processed by RST
Update apt_repository to query if repo is already configured

This adds two dependencies to the apt_repository module:  apt and
apt_pkg.  These come from the package python-apt.  This is used to parse
the host's sources.list and examine whether the repo is already
configured.  This then eliminates the unnecessary
'add-apt-repository --remove' from running.
Update examples/playbooks/ansible_pull.yml

Typo
Merge pull request #1910 from fesplugas/patch-1

Update examples/playbooks/ansible_pull.yml
Merge pull request #1907 from sfromm/issue1888

Update apt_repository to query if repo is already configured
Override Jinja2 Template class to make {% include %} work again

Fixes #1908.
Fix unparseable documentation strings

Fixes #1902.