Visit Generator plugin

[robocoder]

Task: move related .php files from core/Tracker and misc to a plugin.

Part of the misc folder cleanup.

Paves the way to add a UI (change settings #735, warning/progress #693).

[robocoder]

From feedback:

In API documentation, the link "generate some data" (the first one) is not working. It says it requires 'superuser' access.

We should suppress this text if the current user doesn't have superuser access.

[mattab]

Also, before generating any data, the super user should be told that he is going to add fake data in his Piwik which he might not want to.

[mattab]

Rolled up in requirements of #134 new visit generator generating random visits, or possible to generate from an Apache log containing hits to piwik.php?

[robocoder]

Fixed in #1374