Added description for <script> usage.

✓ Code structure copied from idb-keyval

✓ Two new versions: cache-api-keyval-no-fallback.js (688 bytes) and cache-api-keyval-no-fallback-expire.js (461 bytes).

✓ Added: extra small version .silent.js without error reporting and fallback mechanism.

✓ Bugfix: CacheDBFallback not exported by Google Closure Compiler.

✓ Bugfix: expire time not stored.
✓ Bugfix: string to integer conversion in expire check.
✓ Bugfix: key missing in expired check.
✓ Bugfix: prune method not exported by Google Closure Compiler.
✓ Improved: reduced size of date and expire headers.
✓ Improved: do not store date when no expire is used.
✓ Improved: reduced error reporting size.
✓ Improved: make Cache API privacy block check optional while safe.
✓ Improved: renamed db.supported to db.no which is set to int 1 if Cache API is disabled and when no fallback is provided.

• error message in rejected promise for browsers that do not support Cache API.

• added Cache API accessibility test. Some browser privacy settings could block access to Cache API resulting in the SecurityError The operation is not secure error in Firefox. The test consist of a simple caches.open which is very fast.
• data is now always stored in json object data-format.

Improved compression

See description.