setting Access-Control-Allow-Origin header on http api

[MansM]

I was creating a small ui for nomad to allow me to see the status of jobs in a webbased format.
When using jQuery I stumbled on the fact that nomad doesnt set the Access-Control-Allow-Origin header. This results in browser messages like: XMLHttpRequest cannot load http://10.0.50.11:4646/v1/jobs?pretty&_=1453414011020. Response for preflight has invalid HTTP status code 405
index.html:70 ajax error 0

I would probably be able to fix this with using a reverse proxy, but I would like to configure a setting to enable setting the header Access-Control-Allow-Origin: *

[KenVanHoeylandt]

I suggest to also add support for Access-Control-Request-Method and Access-Control-Request-Headers too.
A temporary hack could be just relaying the requested values in the Access-Control-Allow-Methods and Access-Control-Allow-Headers response.

[diptanu]

@MansM @KenVanHoeylandt You should be able to configure the API servers to set arbitrary headers via the agent configuration with this PR. Hoping that would work for you?

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