[cache] Cache interface and in-memory implementation
[cache] README with examples
[cache] Fix typo in ArrayCache::remove()
Update references to 0.3, update lock file
Change react/promise constraints to use tilde operator
Merge remote-tracking branch 'origin/master' into 0.3

* origin/master:
  Change react/promise constraints to use tilde operator

Conflicts:
	composer.lock
	src/React/Dns/composer.json
	src/React/Stream/composer.json
Fugbix typo.

To whom it may concern,

I believe I've stumbled upon an unintentional typographical error in your online documentation. I pray you will find my pull request helpful in rectifying this situation.

Your friend in code,

- Jeff
Update to React/Promise 2.0
Update child repos to PSR-4 for git subs-plit
Move tests to each component
Make components' tests run on their own and from main repo.
Each component has dedicated test config and bootstrap.
Duplication of parts of the skeleton is not ideal, but helps to reduce
dependencies between each test suite. Also, this eases the future
subtree split.