Add method to purge lights without failures

Also well as supporting `delete` method.
Merge branch 'master' into purge-stale

Conflicts:
	lib/stoplight/data_store/memory.rb
	lib/stoplight/data_store/redis.rb
Forward 'delete' and 'purge' from Stoplight
Merge pull request #24 from orgsync/purge-stale

Add mechanism for flushing old lights