PR: Admin add extra fields to sign up. (issue #543)

@davefp I just have done almost all features for this task #543, except feature which allows admins edit PersonMetadata like nested form in rails_admin Person model.
What I just have implement is that there are links to every PersonMetadatum, below screenshot.
![cycki](https://cloud.githubusercontent.com/assets/2162735/2592298/2ca6a492-ba74-11e3-8ff7-74f3554163f9.png)