Set asset bundle name in AssetBundle constructor

Due to default name being used it causes a bit of trouble overriding
when there are multiple assetbundles configured. see
https://github.com/dropwizard/dropwizard/issues/499

Changed to always have an assetname that's not default.
Merge pull request #1 from scooterGrun/dropwizard-0.7

Set asset bundle name in AssetBundle constructor