Don't automatically correct .bowerrc

This commit fixes a few bugs where the location of bower was assumed.  This is now made robust by checking the node_modules directory using the same algorithm `npm(1)` uses.

This also adds an error message if the .bowerrc is not configured correctly:

```
Origami registry is not configured in a .bowerrc file.
Ensure that a .bowerrc has been configured to contain http://registry.origami.ft.com as a search URL
For more information on configuring for the Origami registry see: http://origami.ft.com/docs/developer-guide/building-modules/#set-up-a-bower-package-manifest
For more information on bowerrc files see: http://bower.io/docs/config/
Check Origami registry configuration in bowerrc
```

Closes #19