add token_type param
start implementing token expiration logic
first implementation of token refresh request
have TokenStore handle TokenResponse, rather than Session handling it
refactor TokenStore

We were starting to see real logic entering the mock token store
implemented for testing. That logic has been moved to the new TokenStore,
while the storage backend has been re-implemented with the session
storage interface provided by the Symfony HTTP Foundation component.

refine test for token refresh

consolidate test

remove unneeded lines

clarify intent in test

clean up SessionTest

flesh out expired-token test

test tweak
move TokenRequest classes out of AuthorizationGrant namespace
update README

tweak README
move BearerAuth plugin into project

refactor BearerAuth subscriber
add wiggle room to token expiration
tweak tests
update README

add punctuation
indicate provenance of BearerAuth classes
update README
add missing `use` declarations to README
extract TokenStore from Session constructor into setter
refactor TokenStoreTest
add namespacing for session keys
add error handling information to README
rename TokenRequestErrorException
clarify README