Fix "to" address in SendGrid test source

Can't see a test that confirms this was a deliberately missing `>`
Extend SendGridRequestParser to handle email addresses more robustly

Uses `getaddresses` from stldib's `email` module to extract email addresses from the To, From, CC and BCC header strings. Includes tests.

Currently ONLY returns the email address - discards name details. Does gracefully handle the incorrect address format of a comma-separated name that is _not_ within double quotes (whic the RFC says it should be).
Merge pull request #16 from yunojuno/feature/improve-address-parsing

Improve address parsing for SendGrid backend