plugin.api: Add a validation API.

This API is created to make it easy to validate that we
are fed sane data from the APIs/services used in the plugins.
utils: Add a schema parameter to parse_json/parse_xml.
plugin.api: Add a utils module that contains parse_xml and parse_json.

These imports should be used in the future rather than importing
from livestreamer.utils as that module is not considered part of the
plugin API and may be changed in the future.
plugin.api: Move load_support_plugin out of __init__.
plugins.azubutv: Make use of the validation API.
plugins.gomexp: Make use of the validation API.
plugins.hitbox: Make use of the validation API.
plugins.justintv/twitch: Make use of the validation API.
plugins.livestream: Make use of the validation API.
plugins.mlgtv: Make use of the validation API.
plugins.svtplay: Make use of the validation API.

Also removed support for RTMP streams as they no longer
seems to exist.
plugins.viasat: Make use of the validation API.
plugins.youtube: Make use of the validation API.
plugins.afreecatv: Minor tweaks.
Add plugin for Danmarks Radio (dr.dk).
Add plugin for TV4 Play (tv4play.se).
Add plugin for Aftonbladet (aftonbladet.se).
Merge branch '1.9/validation' into develop
cli: Add a --stream-url option.

This will print a URL to the stream if possible.

Resolves #281.
Merge branch '1.9/stream-url' into develop