Fix issue with refreshed cache entries not having userId info.  The server does not send back an id_token on token refresh requests, so the cache code needs to fill in the new entry with the originating entries userId information.
Return a reasonable error message when an entry cannot be found in the cache during the basic acquireContext flow.
Add some slop, 5 mins, to token expiry calculation to account for clock skew and network latency.
Simplify the code that creates a new cache entry from a refresh response so that it is more understandable.
Make use of CLOCK_BUFFER constant rather than magic number and add a comment explaining it.
Merge pull request #10 from RandalliLama/master

Bug fixes for Issue #7 and #8 plus fix for incorrect caching of refresh tokens.