Add build_configuration_names optional parameter to Xcodeproj::Project constructor
Use a hash instead of an array of strings for build configurations
Do not define default build configurations in the method signature

This will ensure they always get a Release and a Debug configuration and makes it so subclasses don’t need to repeat the default values.
Do not merge a hash with nil
Add spec for extra build configurations parameter
Fix pretty print spec
Document the build_configurations parameter
Merge pull request #82 from CocoaPods/0xced/copy-user_project-build_configurations

Make it possible to pass build configurations to Xcodeproj::Project constructor