Ignore Extra Development Paths
Version bump to 2.15.4
Regenerate gemspec for version 2.15.4
Version bump to 2.15.5
Regenerate gemspec for version 2.15.5
Added Invoice#approve - marks an invoice as approved
Rename delete_or_void_invoice! to change_status!
batch_save should return false on server-side errors.
Fixed NoMethodError if  is nil.
Merge pull request #94 from fimmtiu/batch-save-validation-errors

batch_save should return false on server-side errors.