\SMW\ApiBrowse enables to browse a subject via api.php?action=browse

Code coverage: 100%
CRAP: 22

This is the corresponding Api for the Special:Browse interface (without
support of incoming properties).

In order to work with the Api, the following schema [2] can be used.

- Uses the new SMW\Serializer [1]
- JSON and XML output are supported

[1] https://gerrit.wikimedia.org/r/#/c/89098/

[2] api.php?action=browse&subject=Main_Page

Change-Id: I00c2fab386d0c74ef0b576876edb2f873c17bfde