export GOPATH=$HOME/go
GOPATH是Go的一个系统变量，设置这个是为了指定Go的workspace.
Find more in below page.
https://github.com/golang/go/wiki/SettingGOPATH
export PATH=$HOME/go/bin:$PATH
设置$HOME/go/bin到当前进程的系统运行目录.
Find more in below pages.
https://stackoverflow.com/questions/13795237/what-does-the-export-command-do
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#export
