Only get store if it's needed

I'm trying to dig into #1243 and the inconsistency between belongsToWillChange and belongsToDidChange was irritating me. :)
add inflector rule addition DSL

(@hjdivad / @stefanpenner)
Merge pull request #1256 from nragaz/patch-5

Only get store if it's needed in `belongsToDidChange`
inflector docs
adding unloadAll method for store

also includes test for said method and a test for unloadRecord
Merge pull request #1264 from stefanpenner/inflector-rule-addition-dsl

Inflector rule addition dsl
unload data in store directly
Merge pull request #1265 from iterion/unload-all

Adding unloadAll method for store
Add hook for serialising polymorphicType