Support Nation Builder Custom Fields

[pavpen]

As a 'NationBuilderAPI' user, I would like to be able to read custom fields from Nation Builder, and store objects with custom fields in Nation Builder.

See:

http://nationbuilder.com/when_to_use_custom_fields

This feature was requested by Paul Smith (https://github.com/PaulVSmith).

[pavpen]

The branch implementing custom fields is at:

https://github.com/pavpen/NationBuilderAPI/tree/I-0002_support_custom_fields

The <README.md> there contains an example of using custom fields.