Added documentation for preload parameter

Documented the purpose of the `preload` parameter on the `find` and `findById` functions
Added an example of how to use the preload arg

As per @bmac's suggestion to swipe the example from system/model.js's _preloadData function
Fixed typo

Fixed typo caught by @chrisvariety
name some common anonymous functions
Removed a duplicate explanation of preload

Removed what is essentially a duplicate explanation of `preload` as per @igorT
Updated docs to warn against use of async:true with EmbeddedRecodsMixin
Merge pull request #2198 from ctataryn/patch-1

Added documentation for preload parameter
remove unused variables and enforce in jshint

fixes GH-2209
Merge pull request #2211 from josh-padnick/master

Updated docs to warn against use of async:true with EmbeddedRecodsMixin
Merge branch 'name-some-stuff' of github.com:stefanpenner/data into stefanpenner-name-some-stuff

Conflicts:
	packages/ember-data/lib/system/store.js
Merge branch 'stefanpenner-name-some-stuff'
Merge pull request #2215 from emberjs/unused-vars-jshint

remove unused variables and enforce in jshint
improve error message for push

fixes GH-2201
Merge pull request #2216 from emberjs/2201-data-assertion-message

improve error message for push
Fixed typo

Fixed a typo in rest_serializer#serializeIntoHash
Merge pull request #2220 from nmax/master

Fixed typo
typo in embedded_records_mixin docs
Merge pull request #2225 from James1x0/master

typo in embedded_records_mixin docs
perfs
Merge branch 'master' of github.com:emberjs/data into letsPlayWithTravis

Conflicts:
	packages/ember-data/lib/main.js
	packages/ember-data/lib/system/model/model.js