Better compatibility with Nodemailer mail options. Requires Nodemailer v1.1+.

  * Instead of getting 'from' and 'to' data from the mail object (unparsed, might include mixed strings and arrays), use parsed info from the message info
  * Resolve all streams, urls and file paths to strings or buffers
Merge pull request #5 from andris9/master

Better compatibility with Nodemailer mail options