[#33] Parsed headers support

Uses Mail gem to parse optional headers in the params hash.
Added a Postmark adapter
Merge pull request #38 from r38y/postmark

Adapter for the Postmark Parse API
Merge remote-tracking branch 'origin/pr/34'

Conflicts:
  README.md
Increase minor version for new release of 0.4.0
bugfix with configuration not accepting postmark as an adapter
Update Gemfile.lock

This should have been included in the 0.4.0 tag
Use full email information on Postmarks `to` param

Since Postmark parses the `to` recipients into a new object, let's merge it
again so we have the whole information when setting `config.to` to `:hash`.
Version 0.4.1
wip - Always return an array of recipients for email.to

While this is a change that will break backward compatibility, it is
ultimately beneficial in that it will simplify the implementation of
`EmailProcessor`s, which can now simply loop over the recipients in
`email.to` and check to see if the address is relevant to their app.

This will also fix problems which may have been hidden when `email.to`
returns only one of potentially several addresses, in what may not have
been a valid format depending on an app's `Griddler.configuration.to`
setting.

* Convert all adapters to wrap `normalized_params[:to]` in an array,
  even for a single recipient.
* Add a feature spec for the integration between adapters and
  `Griddler::Email` which ensures that the parsed address is the same
  regardless of email service.
Handle multiple recipients from Cloudmailin

Per Steve at Cloudmailin, header fields are not parsed. This means that
they will simply be comma-separated.
Methods names can be nouns. Weird.
TIL merge doesn't need braces.
Less parameters
Add upgrade text to README, post_install_message
Deprecate :token option for config.to

I am deprecating the token option because with the possibility of
getting multiple recipients it becomes dangerous to operate only on the
token.

Take, for example, an email delivered to `topic.1@griddler.com` and to
`sam@looped-in-to-conversation.com`. With only tokens, the client
application would only see ['sam', 'topic.1'], which could cause
problems.

:token will remain available through the :hash option, which is now the
default option.
Remove trailing '"'
Update README factory example to new recipient api
Merge pull request #51 from brentd/update_readme_for_recipient_changes

Update README factory example to new recipient api
Added '.raw_text' and '.raw_html' to Email

Griddler::Email only supported '.raw_body', which doesn't allow access to both the text and the html of the email easily. Added '.raw_text' and '.raw_html' methods.