Add: use browser caching

[matrixik]

Please check and test Maybe it will be good idea to shorten css and js caching to 7 days instead 30 days. Fix #35

[Mins]

Thanks! I do think its better to use a less aggressive caching as default, so I'll reduce the duration for both.

[matrixik]

Thank you for merging this.

I understood you could prefer shorter caching but setting it to shorter than 7 days will hurt Google Page Speed grade. I think 7 days for css, js and at last 30 days (should be more but oh well...) for all other static content is reasonable.

You should also check your sites with http://gtmetrix.com/

[Mins]

Ah, thanks for linking to that. I had no idea there are recommended guidelines for the caching duration. I went with what I thought was a reasonable default for balancing performance and freshness of data. I'd most likely change it to your recommended values in the next update :)