changed user#random_password

- new random password generation
- moved random_password test to spec
added SystemUser

- sometimes we need an existing user eg. for migrations
- it is possible to execute a block in the context of this SystemUser
fixed failing spec
fixed typo in comment of system_user.rb
more reliable spec for system user
Merge pull request #150 from opf/features/rails3_add_system_user

Features/rails3 add system user