Support for further Datadog sites

[okism]

Hi,
For EU (and all non-US1) based DD customers, BASE_URL should be configurable. Would it be possible to make this change?

Not sure where this all appears, but I found it here:

metrics-datadog/metrics-datadog/src/main/java/org/coursera/metrics/datadog/transport/HttpTransport.java

Line 34 in bb9f580

private final static String BASE_URL = "https://api.datadoghq.com/api/v1";

Thanks for the feedback!