Environment user and password methods

Implemented methods for retrieving the mediawiki user and password from
the current environment context.

Support for `:mediawiki_password_variable`.

Fixed bugs in Environment#lookup and implemented missing specs.

Aliased PageFactory#on_page to #on.

Change-Id: I58b105d36919c251b1ecf28ba21a539f61d03dbc
Helper for user label

Change-Id: I674759e550eed1d0c23963915c742a62336ae53e