Angulartics-piwik plugin added

[florentbruel]

Angulartics does not provide by default the piwik module (see
https://github.com/angulartics/angulartics). angulartics-piwik plugin
bower component added.
Same requirements for other plugins defined in w20 add-on, if this pull request is valide, I can update the issue with all the plugins currently supported by w20-extras / analytics

[adrienlauer]

Since providers are optional, I think we should not force the dependency upon projets using w20-extra. People needing the dependency will have to add it explicitly. Defining the necessary path mapping and shim for those plugins is a good idea though.

We may have a problem with the w20-bridge add-on, where those dependencies will be missing. Maybe we could add them in the bridge bower.json file (https://github.com/seedstack/w20-bridge-addon/blob/master/web-extras/src/main/js/bower.json) so they are readily available for projects using the bridge.

@florentbruel: if you agree, you can do the following things so I can merge this PR:

• Remove the piwik bower dependency,
• Add all necessary code and config (mainly path mapping and shim) for all Angulartics plugins so they can work if they are added explicitly by projects.

Later on, we can add the bower dependencies for plugins in the w20-bridge.

[florentbruel]

@adrienlauer bower dependency to angulartics-piwik has been removed, and all path / shim configurations have been added to ease the provider integration.