Fix link to service template
Added capitalization for Twitter.

I have added the capital T in Twitter as it previously had a lowercase T instead of a capital T.
Merge pull request #535 from wktk/fix-link

Fix link to service template in README.md
Merge pull request #536 from KamranMackey/master

Capitalzied the letter T in Twitter as it was previously lowercase
Add api_key field to tenXer service hook.