blank? and present? commit to return singletons [Xavier Noria & Pavel Pravosud]

The contract of blank? and present? was in principle to return Object, as we
generally do, the test suite and description was consistent with that, but some
examples had comments like "# => true".

This cannot be unclear, we either fix the examples, or update the contract.

Since users may be already assuming singletons due to the examples and the fact
that they were returned before 30ba7ee, the safest option seems to be to revise
the contract and the implementation of String#blank?

The motivation for 30ba7ee was to improve the performance of the predicate, the
refactor based on === is on par regarding speed.

With this commit we start documenting return types using YARD conventions. We
plan to document return types gradually.