Fixed request.post being overwritten globally, added mandrill health check
Changed database healthcheck to use mysql module instead of sequelize.

sequelize is really confusing and the healthcheck seems to be failing
on staging with "undefined is not a function". Hopefully just using
mysql and executing SHOW TABLES as a connection test will fix this.
Merge remote-tracking branch 'erik/mandrill-health-monitor'
Fake S3/email can only be used when NODE_ENV=development.

Also show a warning message if NODE_ENV is unset, and cleaned up README.md
a bit.
Health check subsystems can provide richer data about their status.

Specifically, if they set a 'notes' property on their metadata object,
it will be displayed in the console version of their results, for
easier debugging.