Add Field class

[pezholio]

Fixes #9

@roll, @pwalsh - Does this chime with your expectations in #9?

[coveralls]

Coverage increased (+0.05%) to 98.705% when pulling fbe4764 on feature-add-field into d03825e on master.

[roll]

Thanks!

I added a few comments. And also one thing I think is missing - connection between Schema and Field in methods like schema.fields, schema.get_fields etc.

[roll]

From my experience almost all interactions with schema related to casting go thru schema.cast_row - not sure this cast method needed at all. Also name could be confusing (schema.cast - cast what? schema? data?). WDYT?

[pezholio]

Ah, this was carried over from the old Python API - calling it cast_rows would probably make more sense

[roll]

I suppose it's something like __init__.py in Python? I'm not sure how it works in Ruby but in Python we use this to declare API - adding only public interface and not adding internal stuff (like types after field introduction). Like it's internal API could be changed anytime without any notification etc.

[pezholio]

Yeah, that's it, this is the main file that requires everything needed by the gem. There are other patterns (i.e. we could modularise, so you could only require jsonschema/table or jsonschema/schema), but this is the generally accepted pattern, especially for small, focussed gems.

[coveralls]

Coverage increased (+0.05%) to 98.705% when pulling 98b9a30 on feature-add-field into d03825e on master.

[coveralls]

Coverage increased (+0.05%) to 98.705% when pulling 993c9fe on feature-add-field into d03825e on master.

[pezholio]

@roll The schema and field connections are defined in https://github.com/theodi/jsontableschema.rb/blob/feature-add-field/lib/jsontableschema/model.rb - which is a Ruby module which is included in schema.rb - I've split a lot of things out into modules, which keeps things neater.