Migrate screenstory to webdrivercss

* [X] Capture with webrdrivercss
* [X] Store screenshots in `--screenshot-root` argument path
* [X] Forward to [webdriverxss-adminpanel](https://github.com/webdriverio/webdrivercss-adminpanel)
* [X] Add command line option to use **adminpanel**
* [X] Scope by stories in **adminpanel**
* [X] Add `--screenshot-width` and `--screenshot-orientation` client options
* [X] Screenstory should expects base64 strings with images
* [X] Add generated images to screenstory report
* [X] Automatically load screenstory.yml files
* [X] Use webdrivercss `saveDocumentScreenshot` method
* [X] Update README
* [] -Generate a report with only images that differs.- POSTPONED

Capture With Webdrivercss
-------------------------

Replaced the `capture` function in screenstory extension.

Forward To webdriverxss-adminpanel
----------------------------------

* Add a cli option `--admin-panel` to pass an admin panel url.
* Use this option in screenstory extension to initialize webdrivercss
* Update package.json test script

Scope by stories in **adminpanel**
----------------------------------

Use `--project-name` and story title as prefix.

If no project name is given, nor story title, a default project name is
used.

Generate a report with only images that differs
-----------------------------------------------

A new report should be generated with tests that failed.

It should handle states of image (created, removed, updated)

Add an option `--config-file`
-----------------------------

Load a yaml file with configuration options.
It should accept all the command line available options.

Report update
-------------

Screenstory now fully relies on webdrivercss for screenshots.
It provides updated informations to template. Default image url is:

* diff file when modified
* current when unchanged

Image has following informations:

* browserId:     unique id for given capabilities
* fileDocument:  the path to full document screenshot file
* fileOriginal:  the path to original screenshot file
* fileNew:       the path to new screenshot file
* fileDiff:      the path to new screenshot file
* urlDocument:   the url to original screenshot file
* urlOriginal:   the url to original screenshot file
* urlNew:        the url to new screenshot file
* urlDiff:       the url to new screenshot file
* isNew:         is it a new screenshot for those capabilities
* isModified:    is it a new screenshot for those capabilities
* diff:          webdrivercss response
* capabilities:  the current client capabilities ("client.desiredCapabilities")
* title:         the image human readable title
* story:         the story human readable title
* id:            the image unique id. Maybe best to keep it generated by screenstory.
* storyId:       the story unique id. Maybe best to keep it generated by screenstory.

screenstory.yml
---------------

Define capabilities alias for your project.
Defaults are located in screenstory's root directory.

Added  capabilities for saucelabs and browserstack