I've been working on #31 and have come up with a list of params that adapters should be providing:

* `:to` The recipient field
* `:from` The sender field
* `:subject` Email subject
* `:text` The text body of the email
* `:attachments` (can be an empty array) Array of attachments to the email
* `:headers` (optional) The raw headers of the email
* `:charsets` (optional) A JSON string contiaining the caracter sets of the
  fields extracted from the message

If cloudmailin provides `:headers` and `:charsets`, would you mind including them in the `normalized_params`?

It would also be a good idea in this case to start with a fresh hash, so that we don't have the original params included here.