Fix SendGrid references in README.md

- Corrected capitalization.
- Fixed a broken link.
- Won the internet.
Remove unused charsets tempvar

It looks like earlier versions (such as 1320736b) of #extract_body were
using Iconv and needed the charsets, but the latest version was no
longer using the tempvar for anything
Merge pull request #11 from kevinthompson/patch-1

Fix SendGrid references in README.md
Merge pull request #13 from bemurphy/remove_unused_tempvar

Remove unused charsets tempvar