Re-add optional Browscap integration in 3.x version

Problem/Motivation

The 7.x-2.x release series of this module optionally integrated with the Browscap module → to provide more context on the end-user's browser, which is useful for debugging browser-compatibility issues.

Browscap integration was dropped in the 3.x release series of this module.

Proposed resolution

Add back the Browscap integration. Make it optional as before.

Remaining tasks

1. Write a patch
2. Review and feedback
3. RTBC and feedback
4. Commit

User interface changes

Unknown, hopefully none.

API changes

Unknown.

Data model changes

Unknown.