Add eredis_sentinel_client/masters auxilary modules
Add eredis_sentinel main module with test specs
Add sentinel support to eredis_client
Add check that sentinel is installed on the system or skip sentinel tests
Add sentinel chapter to README.md
Fix remove stalled pids from subscriber list of sentinel master
Clean up sentinel part in README.md
Merge branch 'sentinel' of https://github.com/mkurkov/eredis into update_deprecations

Conflicts:
	README.md
	src/eredis_client.erl