Move export endpoint to a separate gem

[PerfectlyNormal]

As mentioned by @derekprior on the commit, having the endpoint included inside this gem adds an extra dependency, and might not be the best place for it. Should get this moved out after all.

[findchris]

Is @derekprior working on a corresponding gem?

[derekprior]

Nope. I just wanted to use highcharts-rails without the dependency on cocaine.

[findchris]

Yeah, cocaine dependency is a terrible thing. ;-)

[PerfectlyNormal]

Discovered highchart-image-api today. Looks to be usable, and instead of creating a mostly duplicate gem, I'm leaning towards just using that

[PerfectlyNormal]

This issue has been here four and a half years now. Let's face it, it's not happening.