modify $PATH instead of definibg aliases

After `goon` command the $PATH will looks like this

    $ echo $GOPATH
    /Users/aeg/.goenv/gosample:/Users/aeg/Devel/gosample
    $ echo $PATH
    /Users/aeg/.goenv/go-1.1.2/go/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

And it will be transparent to use `go` tool

    $ which go
    /Users/aeg/.goenv/go-1.1.2/go/bin/go

And this meets the reccomendations about [The GOPATH environment variable](http://golang.org/doc/code.html)