save npm original package maintainers to npm_module_maintainer table. 

fixed #464

from 1.5.x upgrade to 1.6.x, execute [v1.6.x-upgrade.sql](https://github.com/cnpm/cnpmjs.org/blob/26efecd2fc3b541088f2720beccec6d31b0e7878/docs/update_sqls/v1.6.x-upgrade.sql) first.
npm 上应该是没有区分 author 和 maintainer 的吧，这里是只取了 author 的？