use commander and other npm modules to refactor nrm.
1. auto output help infomation
2. use npmconf instead of using spwan to save npm registry config
3. save user's custom registry info to $HOME/.nrmrc
4. only allow to delete custom registry config
update readme
add home command
fix typo err
extend empty object
refactor nrm with mature modules
add response time check
show current registry when check response time
add taobao registry
Merge pull request #5 from Jeremial/master

show current registry when check response time
update version
add `rm` as alias for `del`
Merge pull request #6 from weakish/rm-alias

add `rm` as alias for `del`