Introduce AngularJS to Piwik, first step: Website Selector rewrite #4690

This is the email kickstart of the new AngularJS adventure! We'll do our best to make the most out of AngularJS & work as a team to make this a positive change. Thomas will get started on Benaka's suggestion.

Goal: integrate the website selector widget as an example of how to best use AngularJS.

It should have the following requirements:

1. the selector should be in a self-contained class/widget/whatever
2. the selector should be re-usable; specifically it should be possible to place it in another template and place two or more in one page (the selector is used in a couple places, so there'll be ways to test this)
3. the selector should be customizable
4. the selector should be documented and it should be possible (at least conceptually) to expose this documentation to plugin developers
5. ensure the selector's resources are cleaned up properly on page change, widget reload, etc.
6. Untrusted input should be escaped but not by Piwik itself (for example w/ jquery: $('<span/>').text(site.name); instead of $('<span>' + site.name + '</span>');).
7. it should be possible to create another class/widget/whatever that extends the site selector

once this is done, a set of patterns for using angular-js in Piwik should be observable.

In ad29373c4f82738386ee3e60f5b2f9b5215882b9: refs #4690 added documentation

This is done except for part 4 exposing documentation to developer.piwik.org. This is to be defined and listed in #4691