Parser improvements (continuation)

* Improved `.gitignore` so it match only top directories, not any other files located elsewhere
* Replaced `rcov` to `SimpleCov`, because it doesn't support Ruby >= 1.9
* Improved Rakefile so it's platform-independent
* `:guess` option accepts: `:begin`, `:middle`, `:end`, because most people wouldn't expect time `12:00` when specifying only day, but would rather want `00:00` so now there's such option
* Additions to Date and Time classes