[Relation] Supports HasOne relation

Description / Steps to reproduce / Feature proposal

Add support for HasOne relation between two models.

Update factory method to handle this relation
create interface for CRUD methods available for the relation. See this as an example.
- create default class which implements the interface
Implement the hasOne decorator
- model decorator: infers the metadata
create test cases
update docs
create a follow up issue to demonstrate using multiple relations in a new/existing example

@elv1s

Cross posting in #1449 (comment) from @elv1s:

We have a requirement for hasOne in addition to hasMany and belongsTo

Currently hasOne is not part of our Q1 roadmap. So I have 2 questions:

@elv1s, what is the timeline you're thinking in terms of having hasOne support?
@strongloop/sq-lb-apex @raymondfeng @bajtos, after we have the 2 model relations, do you think it's a relatively small effort for us to get hasOne in?
EDIT: I realized that we've estimated this task already. Given what we know now (esp when implementing BelongsTo, are we still good with this estimate?

We are developing a POC with LB3 right now. If we move to LB4 we would need hasOne relationships early 2019.

Closing as done.