replace mapProperty with mapBy

just a detail, but as I understood, the mapProperty has been quietly deprecated in favor of mapBy
DS.attr doc
Added docs to show how you can replace the RESTAdapter
Fix capitalization of RESTAdapter in Adapter documentation
Merge pull request #1182 from bradleypriest/ds-attr

DS.attr documentation
Merge pull request #1186 from nragaz/patch-1

Fix capitalization of RESTAdapter in Adapter documentation
Merge pull request #1181 from sly7-7/patch-1

replace mapProperty with mapBy
Merge pull request #1185 from toranb/master

Added docs to show how you can replace the RESTAdapter
Remove unnecessary .extend()