feat(HostInfo): adds login.defs plugin

Adds a login plugin that opens `/etc/login.defs` and maps definitions.

https://github.com/rackerlabs/ohai-plugins/blob/master/plugins/login.rb
the "io" library is used for lua style synchronous i/o.  please use the luvit "fs" library instead.  something like this: https://github.com/luvit/luvit/blob/master/examples/fs-basic.lua#L3