Implement v3 support with minimal impact on current use

- Outsource the construction of the URL by introducing self.api_path
  which is built at instanciation of a controller.
- Currently assume v2 to be standard behaviour as it was before.
  Thus there won't be any breakage for current scripts with a v2 controller.
- Update documentation with examples for both v3 use and how to migration from v2.
Update sample scripts for v3 support and their documentation

Now that we support v3, we should update the sample scripts to reflect that.

The documentation shows examples for v3, though v2 usage should remain working,
as long as v2 is the assumed default version.
Merge branch 'v3-support' of git://github.com/matsimon/unifi-api into matsimon-v3-support

* 'v3-support' of git://github.com/matsimon/unifi-api:
  Update sample scripts for v3 support and their documentation
  Implement v3 support with minimal impact on current use

Conflicts:
	unifi/controller.py