Add a method to get all Items in a thread safe manner.

[hikhvar]

As discussed in Issue #36 I needed a way to get all not expired items. This PR add such a method. Test and Benchmarks are included. Feel free to make any suggestions.

[totallyunknown]

Any chance to get this included?

[patrickmn]

Thank you for this. I modified Items() to return a new map with unexpired items https://github.com/patrickmn/go-cache/commit/a2d8b56f0c21c5562c23717b6137ed6254ddc6d4

Would you be willing to change this PR to add your test against Items() and adding yourself to CONTRIBUTORS?

[hikhvar]

Hey, I updated the pull request.

[pablodz]

Found this fork: https://github.com/arp242/zcache