Make gin work with graceful

[simonlei]

Use graceful(https://github.com/stretchr/graceful), we can shutdown the application gracefully.
Gin kill the app in 3seconds(in lib/runner.go), while the connection is still living.
And, the lib/builder.go use go to build binary file, but if the old process is living, the build will fail.
So, I think gin can build the binary to a temp file, then use install instead of copy/mv to overwrite
the binary file. So that We can use gin to work gracefully with graceful.

Of course, install can only used in *nix...