Features: enable/disable methods, unit tests

Summary:
Features::enable($feature) and Features::disable($feature);
now allows to edit configuration at runtime.

This will be convenient for integration tests depending of
some feature.

We add tests covering the whole Features class.

Test Plan: arc unit

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D215