Capybara assertions refactoring

Hi @wojtekmach,
I'm back again ;)

I have refactored capybara/assertions.rb this time, all tests are passing.
I have also addded a TODO as you might read along the code.

I hope this could get merged as it makes it much easier to understand and
removed a lot of duplication, although we still have some repetition of local vars.

I will do another refactoring soon, so far this is what I have got.

With regards,
Nalesso Moreira