Better errors and allow updates

[ghost]

This started as a response to Issue #5 and ended up including some other features that I find useful for my own projects:

• Show meaningful errors when uploading data with validation problems
• Allow mass updating in addition to mass creation
• Clean up the upload form

[karlwilbur]

Rather that override create! it seems that this would be a good candidate to utilize the passed in &block ...something like:

active_admin_importable do |model, hash|
  object = model.find_by(id: row[:id]) || model.new
  model.column_names.each do |key|
    key = key.to_sym
    if hash.keys.include?(key)
      value = hash[key]
      object.send "#{key}=", value if value != nil
    end
  end
  object.save ? true : object.errors
end