splits redmine.rb into initializers
Merge pull request #2349 from opf/feature/restructure_redmine.rb

splits redmine.rb into initializers
fix aglio warning
minor changes to user spec

- add already implemented self-link
- rename mail -> email
- fix examples

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add group endpoint

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add cross-references betwen users and groups

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fix description

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specify assignee/responsible as user+group

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specify multiple types for a single property

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