Add proxy support, for sending events via a CNAME

[masojus]

HTTP Proxy support via KeenClient.setProxy() is available as of commits 4613656 and bfb7cad. Is there more that needs to be done here?

• One thing I noticed is that KeenQueryClient does not provide the same functionality.
• Related: Issue #12, PR #13, and https://github.com/keenlabs/KeenClient-Java#using-an-http-proxy
• However, as mentioned in Issue #12, the readme should mention that for authenticated proxies one must use Authenticator.setDefault() to set an Authenticator that will return the credentials required.

[masojus]

We need to support setting the root authority to something other than https://api.keen.io. Adding support for true HTTP proxying to query functionality and clearing up authenticated proxy support in the docs should/will be a separate issues. But, using an alternate base address is already supported for KeenClient via the setBaseUrl() method. It needs testing, and possibly work to make sure it applies to the KeenQueryClient as appropriate. This way a given client can leverage the custom SSL domains feature. This should work with HTTP proxying as well, of course.