Merge pull request #3 from scorphus/master

Improve existing documentation and add documentation for grace period
Removing trailing space
Locking storage
Merge pull request #5 from marcelometal/lock

Locking storage
Add, improve and amend some tests (re #5 and #7)
Support expiring a given material (fix #6)

Also, have RedisStorage to use redis for expiration times (fix #4)
Release 0.5.0
Check if expired when deleted
Log some info while running