[WIP] implement basic rate limiting

[fairingrey]

Addresses #31

This uses rocket-governor to implement rate limiting.

[sbihel]

I fail to see the benefits of doing rate limiting in the app, instead of at the load-balancer/proxy level. Do you think it would be beneficial to have both?

[fairingrey]

@sbihel I agree more or less (that rate limiting could simply be achieved by putting a load balancer or etc in front) -- this is definitely something up for discussion. The way it's filed @wyc did seem to want rate limiting as an option in the rocket app itself.

If we decide this is not needed I can close this PR, but it doesn't seem difficult to add if needed.

[sbihel]

Will close this but keep it in mind as an option. Will rephrase the issue.