[#18] Added "boolean" field
Added IntegerField
[#18] Replaced deprecated methods to_dict() and from_dict() with serialize() and __init__()
[#18] Now using specialized comparison for fields too

The is_equivalent() method is used, in order to make it possible
to recursively ignore keys in sub-objects as well..
[#18] CkanDataset.private is now a boolean field
[#18] Added rich comparison for ResourcesField
[#18] id field of dataset/resource now marked as key
[#18] CkanResource.position marked as integer, but commented out as it is generated
[#18] Base fields now using default implementation of serialize()
[#18] ``None`` is now an acceptable value for non-required string fields

Updated tests to check for ``None`` instead of ``''`` when the field is omitted
[#14] Tests for dataset creation via high-level API (fixes #9)