Add eu-west-1 as a valid location constraint

It's an alias for 'EU':
http://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region

'EU' is the only constraint that's not the the name of a region, so I
tried to use 'eu-west-1' the first time.
Merge pull request #35 from greenalto/eu_west_1

Add support for eu-west-1 as a location constraint