adding the fuubar gem to have a better rspec progess bar.
extracting UserMixins::Bv out.

I hoped that by converting a small part of the specs to the new rspec syntax would
help solving the mystery of some specs failing when run together with other specs
while passing in isolation.

But it makes no difference to use the new syntax. Thus, migrating to the new
rspec syntax is an independent task.

trello:
* https://trello.com/c/Dlio0wG9/281-rails-4
* https://trello.com/c/zBd0dHaj/620-test-suite-uberarbeiten
renaming UserMixins::Bv to UserMixins::Bvs in order to avoid naming conflict with Bv class.
specs: clearing the cache before each spec.