Document general helper functions in utils.
Document address formatting and delivery class utilities.
Document extendable classes and base error classes.
Remove utils.noop and utils.deep_equals and replace them with _.identity and _.isEqual respectively.
Fix article in vumi_utc doc string.
Mention that address types are Contact object fields and warn about use of infer_addr_type.
Add utils docs to documentation index.
Merge branch 'feature/issue-162-add-docs-for-utils' into develop