Moved "bio" back to "person", but with type-specific storage. People can fight over where such things belong on another day, for now keeping it in "person" has the advantage that the Tcl API is 100% unchanged so moving it to type-specific storage won't break .LRN.
Added attributes for "user" columns "screen_name" and "username", as well as the new "bio" column which replaces the old generic storage attribute assigned to "person".