Add Cache-Control header

[Antolius]

Add Cache-Control response header for static files in order to improve page loading speed.
Fixes #322

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[Antolius]

Hi! I'm new to the code-base, so might have missed something obvious, but this seems rather straightforward.

I've also noticed a TODO comment on the InitStaticRoutes method that says it should be converted into a func. If you'd like I can just update the method into something like this:

func InitStaticRoutes(app *App, r *mux.Router)

and leave its implementation as-is.

Or, if the comment had some broader implications I could try working on them as well. For example, it seems like shttp field in the App struct could be removed, in which case the InitStaticRoutes function would not need to modify the app passed into it. This might be a bit bigger change, so it might be better to have it in a separate pull request.

[thebaer]

Thanks for looking at this! And sorry for not responding earlier.

Overall, everything looks great! I have just a small suggestion to help keep things organized.

Otherwise, I really appreciate you including a test with these changes. As for that TODO and getting rid of the shttp field, please feel free to submit a separate PR with those changes -- we'd very much welcome those improvements!

[thebaer]

Awesome, thanks!