Add Cache-Control header

[toshinari123]

fixed issue #11

[kiootic]

@toshinari123

• The indentation for Go uses tab instead of spaces, please run the Go formatter when you edit the code.
• The compiled binary is accidentally committed into the repo.

[limouren]

Just passing by, cache control should be configurable?

[kiootic]

This is just a reasonable default for Cache-Control header. Customization of headers would be handled separately.

[limouren]

icic. It makes sense.