Define a mixin instead of monkeypatching Object class

Now Fuckyeah doesn't monkeypatch Object class. Instead, it provides you
with Fuckyeah module that you can include to a class to use the power of
its helpers.

The second change: now #fuckyeah method returns a copy of the object,
and #fuckyeah! returns the same object (dangerous!). This API change was
made for consistent exclamation sign usage within Ruby community.

Specification has migrated to RSpec. RSpec is a powerful tool, and now
our specification code takes less LOCs.

This release is bumped (MAJOR == 3, MINOR == 0, PATCH == 0).