HTTP Basic Auth for POST /measurements

# HTTP Basic Auth for /measurements

Does what it says on the box!

```
$ htpasswd -bn canaryd changeme
canaryd:$apr1$5L6N4DMG$DUvhIPK5Suj7X5E77RGTA.

$ $GOPATH/bin/godep go run canaryd.go --http_basic_username='canaryd' --http_basic_password='$apr1$5L6N4DMG$DUvhIPK5Suj7X5E77RGTA.' --http_basic_realm='http://localhost:5000' &
[1] 92567
2014/05/11 21:05:51 fn=main listening=true port=5000

$ curl -d @data.json -X POST http://localhost:5000/measurements
401 Unauthorized

$ curl -d @data.json --user canaryd:changeme -X POST http://localhost:5000/measurements
2014/05/11 21:06:07 fn=post_measurements count=1

$ curl -d @data.json --user fake:notreal -X POST http://localhost:5000/measurements
401 Unauthorized
```