add user doc for directory view
start documentation for xivo-dird
dird: improve rest_api config

- remove static_folder config
- move wsgi_socket to rest_api config
remove http from service plugin

reason: http will be only available to view plugins
dird: add unload() in service plugin
remove the name field from the service plugin example

the name field will be injected by the plugin_manager based on the
name in setup.py
dird: update CLI arguments (add -d and -l)
some configuration file refactoring for xivo-dird
dird: split enabled_plugins section
dird: add return value from service.load
rename the columns_maps to source_to_display_columns option
add some doc for view plugins
add a plugin section in the xivo-dird page

add a quick overview of the plugin architecture in the top of the docuemntation
with links to each section for an in depth description of each extension point.
move the api documentation to the dird page

this might not be the final organisation for the dird documentation I think
regrouping all xivo-dird related stuff under system/xivo-dird and making
links in other sections would be a good way to organise this documentation.
add some colors to code-blocks
fix some typos
bump version to 14.21
archive upgrade: make sure xivo-upgrade is in the right version

reason: the admin may have run xivo-upgrade before, and cancelled the
upgrade when he saw the prompt to upgrade to the latest version.
archive upgrade: use apt-pinning for 13.25+
archive upgrade: simplify procedures

reason: apt-pinning is a better way to circumvent the bug #5087