Missing dependencies in install instructions

When following the setup instructions for installing the web server, I ran into errors related to missing dependencies. Here's an example of the errors on a completely fresh install (go version 1.3.3):

```sh
$ export GOPATH=/tmp/freegeoip
$ go get github.com/fiorix/freegeoip
$ go install github.com/fiorix/freegeoip/cmd/freegeoip
/tmp/freegeoip/src/github.com/fiorix/freegeoip/cmd/freegeoip/main.go:20:2: cannot find package "github.com/fiorix/go-redis/redis" in any of:
	/opt/local/go/src/pkg/github.com/fiorix/go-redis/redis (from $GOROOT)
	/tmp/freegeoip/src/github.com/fiorix/go-redis/redis (from $GOPATH)
/tmp/freegeoip/src/github.com/fiorix/freegeoip/cmd/freegeoip/main.go:21:2: cannot find package "github.com/gorilla/context" in any of:
	/opt/local/go/src/pkg/github.com/gorilla/context (from $GOROOT)
	/tmp/freegeoip/src/github.com/gorilla/context (from $GOPATH)
```

I was able to fix this by explicitly getting the `go-redis` and `context` packages before running the install command. This pull request simply updates the readme with these additional commands to get those packages.

However, I'm new to Go, so if there's a way to setup those packages as dependencies so they automatically get fetched, that might be preferable, but I at least wanted to raise the issue for anyone else that might be running into this on a fresh install.

Thanks for the great package!