Skip fields explicitly & refactoring

If you want to explicitly skip parsing of a certain field, you can force this by
using the new `skip!` dsl-method like this:

``` ruby
class Superhero

  attr_accessor :name, :strength

  parseable do
    text :name
    skip! :strength
  end

end
```

Made several improvements:
- Moved first part of field parsing to FieldParser class
- Created seperate parser classes for each datatype
- Moved creation of specific parsers to new ParseableAttribute
- Moved instance methods to separate class.
- Divided class methods into Dsl and Parsing methods.
- And many other small refactorings...