fix some specs
add docs for tracking
change how sendgrid content works to behave like mailgun, fix content and files serialization (hash not array)
add comments
make all mailgun images inline
make service into a module
add sender::base back, now that we have shared behavior
test sender::base
implement tracking
more informative exception messages
change tracking api
add specs for new tracking code
make more specific exception classes
more specific postmark errors
more specific mailgun errors