master: enable gzip compression for POST responses

This will allow POST /hosts/statuses responses to be compressed, which
should speed up things on slow connections (e.g. VPN). This would be
welcome as the statuses for all hosts in our current test environment
exceeds 1MB uncompressed text.
client: accept gzip compression

Allow for api requests made by the helios client to be gzip compressed.
Merge pull request #320 from spotify/dano/gzip

use gzip compression for API requests
master: enable CORS

Lets browsers make requests to the helios api.