Refactoring the internal structure

The plan is this:

- `/bin/xcpretty` deals with CLI and instantiates a `XCPretty::Printer` with right parameters
- printer is only one, and `Rspec, Simple, SomeCoolOutput` are becoming `Formatter`s
- we pull out regexes in `Tokenizer` that deals with `stdin` and calls methods on formatters
- that way no regex should be needed in formatters, they'd only respond to methods like

``` ruby
def print_passing_test(test_case, time)
  format_test("Check this #{test_case} out! It ran in #{time}")
end
```