import_builtin_asset.ASSET_NAME

Adds direct namespacing of builtin assets. With this PR the following two calls are equivalent:
```python
menpo.io.import_builtin_asset.breakingbad_jpg()
menpo.io.import_builtin_asset('breakingbad_jpg')
```
benefit being the top variant is tab-completable. The string variant is still allowed, just adds an alternative for use in notebook sessions. Note that all periods in built-in assets are replaced with underscores for the method-variant (as can be seen with the breaking bad example above).