Better management of custom user_resources

[piotaixr]

Fixes #35

Also this changes how admin_user_resource default is set. This avoids verbose config.

[coveralls]

Pull Request Test Coverage Report for Build 111

• 101 of 105 (96.19%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 93.545%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
spec/dummy/app/models/another_user.rb	5	9	55.56%
<!--	Total:	101	105	96.19%	-->

Totals
Change from base Build 109:	0.5%
Covered Lines:	913
Relevant Lines:	976

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💛 - Coveralls

[michelson]

thanks @piotaixr ! sorry for the delay, it's merged on master