Version 0.4.0

Many new features have been shipped in this release!

1. You can now:
   • check whether the cache contains something (2 APIs)
   • iterate through the cache (4 APIs)
   • remove something from the cache (on-demand partial cache clearing)
2. SyntaxWarnings can be suppressed now
3. API documentations are improved with examples
4. Minor bug fixes
   • #8 Do not require the key maker to have the same signature
   • #10 Caching functions without arguments
   • #14 Partial cache clearing