Extend PHP client to add caching and authentication

[ken-duck]

Add a couple of features to the MVP client

Caching

• Cache results to disk for 12 hours, so if the same package is scanned more than once it does not check OSS Index
• Before requesting results from OSS Index for a package, see if the cached results are valid
• If a plug-and-play caching algorithm cannot be found, do not implement caching

Rate limits

• Reads an optional username/token from a configuration file on disk which is passed via the OSS Index API to allow use of an authenticated request which has a less restrictive rate limit
• Gracefully handles a rate limit by informing the user that it has be exceeded
  • If the user is unauthenticated, mention that authenticated users have a higher rate limit