Adding readable functionality to the format.
It is now possible to call the readable function to get back a map representation of the given interval.
Changing the method name to be more clear. Adding the implementation for the date time type.
Fixing the docstring. Had wrong information about the returned map.
Fixing the docstring. Replacing the word interval to instant.
Changing from multimethods to protocols.
Merge pull request #55 from thiagotnunes/master

Adding readable function to convert a given interval to a map representation using the given fields.