Add WorkerChange interface

Add WorkerChange interface

WorkerChange interface defines API for implementation to be informed
about changes in workers.

The mock implementation of interface is provided in workers package.
It is generated using mockgen command:

mockgen	-package workers \
	-destination=workers/workerchange_mock_test.go \
	-write_package_comment=false \
	git.tizen.org/tools/boruta/workers WorkerChange

Change-Id: Ia17aefd747057868bf2b4b57332397a591ac038a
Signed-off-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>