Use "!" as separator.

Merge pull request #17 from Nyholm/separator

Changed separator to exclamation mark. Fix #16 #15

Better tag keys

Changes:

• Renamed getTagItem to getItemWithoutGenerateCacheKey
• Validate tag name
• Added TaggableItemInterface
• No need to update keys if no tags are used

Allow php 5.4 and doc update

Merge pull request #8 from Nyholm/docs

Improved docs how you implement the trait

Docs and codestyle

This release has docs and code style fixes. It also adds a dependency on the stable psr-6 package.