Add ability to embed graphite graphs in emails

Change
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This adds a new config option to SMTP_OPTS, 'embed-images' which is a bool. When true, it will download the image content from the graphite host and embed it in the alert email. If fetching the content fails or this setting is false, the old behavior is used.

Rationale
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Our organization uses GMail (apps for your domain), which attempts to fetch and cache the images from our graphite host, which is on our company intranet. This results in every graph being a broken image. Additionally, this removes the requirement that recipients be on our company VPN in order to see the graphs (exceptionally helpful on mobile devices).