Mail documentation fixes and code tweaks

The documentation has `BW::Mail.compose{...} do |result, error| end` instead of `BW::Mail.compose(...) do |result, error| end`. This should be fixed on the bubblewrap.io site as well.

I also reformatted how the mail class handles default values for the options that it is passed. Cleaner and more readable/understandable code is the result!