[prometheus-to-sd] Support for SUMMARY metric types

It seems that prometheus-to-sd is unable to translate summary metrics and throws errors every time that scrape the metrics:
```
W0817 15:04:56.690842 1 translator.go:61] Error while processing metric http_request_duration_microseconds: Metric type SUMMARY of family http_request_duration_microseconds not supported
```
However, the [stackdriver-prometheus](https://github.com/Stackdriver/stackdriver-prometheus) project actually is able to parse this kind of metric and push it to stackdriver (https://github.com/Stackdriver/stackdriver-prometheus/blob/9f5c836bfc3abea91b7473f4a57c107806099bf9/stackdriver/translate.go#L118).

Is there any reason that prometheus-to-sd does not support this kind of metrics? Are you planning to add support for summary metrics? I can work on it if you accept PRs.