Implement `CommaSeparatedEmailField`
Add CC and BCC migration
Send email to multiple recipients; add CC and BCC.

This is an intermediate commit where `send` always returns a list with only one `Email`. TODO: modify it to return the `Email` directly.
Make `send` return `Email` instead of list.
Rename `to` back to `recipients` for backward compatibility.
Move email list creation logic in `CommaSeparatedEmailField`.
Purge mutable default parameters.
Merge pull request #63 from stefan-mihaila/master

Send single email to multiple recipients; add CC and BCC.