+ caching: add a size method to Cache[V]

Fix for #476

The scaladoc reflects that the size may return an upper bound instead of the real current number of unexpired cache entries.
Merge pull request #480 from agemooij/issue-476

+ caching: add a size method to Cache[V]
= can: be more resilient against stray `Terminated` messages in HttpHostConnectionSlot, fixes #459

Accomplished by introducing a new state just for waiting for connection
termination.
Merge pull request #482 from jrudolph/i/459/fix-host-connector-not-expecting-connection-termination

= can: be more resilient against stray `Terminated` messages in HttpHostConnectionSlot, fixes #459
= io: more verbose error reporting in case of SSLException, fixes #470
Merge pull request #483 from jrudolph/i/470/improve-ssl-exception-errors

= io: more verbose error reporting in case of SSLException, fixes #470
Added support to play-json with basic error handling.