Introduction of cache interface and a file cacher.
Authorized transport should read from cache and write to cache.
Merge branch 'cache' of github.com:rakyll/oauth2 into cache
License fix.
Moving error handlers to the cache implementations.
Fixing styling issues for caching.
Cache read operation should be handled during construction.
Provide a transport constructor for transporters with a cache.
JWTConfig should provide a cached transport constructor.
Provide missing docs for Cache interface and transport constructors.