Explicitly define a bower install directory

bower install will put its files into a folder specified by the first .bowerrc file it finds in or above the project root. It's possible for bower to install its files in a place we do not expect without this explicit declaration.
Merge pull request #1837 from CodeOfficer/add-bowerrc-file

Explicitly define a bower install directory
Fix error assertion's message

This fixes the error message to reflect the changes described in [Defining Relationships in Models](https://github.com/emberjs/data/blob/master/TRANSITION.md#defining-relationships-in-models).
Further improve message
Merge branch 'patch-1' of https://github.com/lulezi/data into patch-1

* 'patch-1' of https://github.com/lulezi/data:
  Further improve message
  Fix error assertion's message