Active label API Proposals

[jorroll]

After working on the neo4j-core .union patch, I wanted to keep up the momentum. This PR contains proposed ActiveLabel API updates. The main change from what you had (I think) is that I've separated ActiveLabel modules, which simply respond to labels, from how labels are added to nodes.

I've also made a few other changes, which were the result of thinking through the implementation process, and realizing that I don't think some of my previous ideas were readily implementable. (for example, if Actor is an ActiveLabel module, in order to allow Actor.acted_in queries, I don't think the has_many :out, :acted_in association can be called inside a included do block, but rather should be called directly on the Actor module. Ideally, I'd have liked it to be called inside an included do block to mimic ActiveSupport::Concern)

Note: It will probably be easiest to read over if you first read the "files changed" as rendered markdown, without looking at the comments, and then afterwards go back and view the comments.

[codecov-io]

Codecov Report

Merging #1458 into active_label will increase coverage by 0.02%. The diff coverage is n/a.

@@               Coverage Diff                @@
##           active_label    #1458      +/-   ##
================================================
+ Coverage         96.84%   96.87%   +0.02%     
================================================
  Files               205      205              
  Lines             12603    12600       -3     
================================================
  Hits              12206    12206              
+ Misses              397      394       -3

Impacted Files	Coverage Δ
lib/neo4j/tasks/migration.rake	32.14% <0%> (+0.83%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e0582e4...8f5b1ec. Read the comment docs.