Document the fact that TO accepts arrays of hashes

It's utterly not clear with the current documentation, 
despite a pretty clear example using `#map`.
Update README to match improved `TO` documentation.

I also took the opportunity to clean up the example by extracting the
`#map` call outside the `mandrill_mail` call for clarity, reword a
weird reference to "merge tags" and the `:vars` explanation.
Merge pull request #47 from olivierlacan/patch-2

Document the fact that TO accepts arrays of hashes