Collection::add() and Collection::remove() methods

[adambiggs]

This pull request adds methods to add/remove records from a Collection without affecting other records in the Collection.

Turns this:

photo = album.photos().create()
photo.updateAttribute 'album_id', null
photo.updateAttribute 'album_id', album.id

Into this:

photo = album.photos().create()
album.photos().remove photo
album.photos().add photo

[aeischeid]

we gotta update the spine.site to cover all this relation stuff you have been doing!

[adambiggs]

Agreed! More than happy to help write docs, but I'm waiting to see what happens with #433.

Wouldn't want to update spine.site and then have to rewrite the same docs again somewhere else :)

[aeischeid]

Yeah. repeating work sucks. However, it is probably going to be at least a few weeks before I am even going to consider rewriting the docs app, so if you have time to kill I think most of the work would be relatively portable if we are just adding documentation on the markdown pages of current spine.site. Your call :)

[adambiggs]

Good point, the markdown will be the same either way. I'll write some docs next chance I get.