Update key section to define minimum standards.
Whitespace fixes.
Reivse the 'data' section for greater precision.
Revise Pool/Item definitions.
Add definition of hit; revise miss definition.
Correct interface names.
Clean up language for PoolInterface; improve documentation.
Add reference to RFC 2119.
Grammatical and typographic improvements.
Clarify handling of expiration times by Implementing Libraries.
Clarify references to serialize()/unserialize().
Require getItems() to return an associative array.
Improve language and completeness around the ItemInterface.
Strengthen details around TTL expectations in the set() method.