Implement eager loading

[Tape]

Description

Here's a simple implementation of eager loading (NOTE: I am not familiar with MongoDB so this will need to be added or refactored in a way that Mongo can handle it, I left relevant TODO comments on what to remove). Autofetch on large queries can be quite detrimental and this should help remedy the problem. This should also resolve #389, however bulk updates should be added.

How it works

Assume I have a database table with the models Image and Tag with a relationship Image.hasMany("tags", Tag). If I do Image.find(...).run(...) currently it will do n+1 queries (query for images, n queries for tags). The implementation I have provided will only perform two queries to fetch the data. In order to use it, autofetch can be left off and do Image.find(...).eager("tags").run(...). The returned result will be the same, but will be far more performant.

[vendethiel]

👍 good job :)

[HMCardcash]

Very nice, thanks mate!

[dxg]

Really nice, passing tests and all.
I'd like to merge this, but want some input from @dresende given it's significant new functionality..

[dresende]

I like it :)