Coerce validation intervals to int's
Avoid serializing YAML for blank values

When dumping to the database, ActiveRecord would run blank values (nil
and "") through the serializer and wrap it a YAML string that is not
valid for us to parse as a schedule. When the attribute is blank going
into the serialized format, it should remain blank coming back out.

Fixed #231
Cleanup
Merge pull request #237 from avit/issues/231-serialization

Fix blank YAML serialization

Fixed #231
Enable support for ISO 8601 deserialization

Makes it easier to work with JSON representations of schedules.
Make sure time is required in and add some specs for iso dates
Use a case statement instead
add interval validation for all rules
run interval validations on validate
Merge pull request #238 from donaldpiret/iso8601-dates

Support deserializing values in ISO-8601 format
refactor interval validation
YearlyInterval#interval should return self like other such methods
Merge pull request #236 from ajsharp/interval-type-coercion

Normalize and check intervals as positive integers
Bump version
Cleanup and whitespace