Fix supervisord collector (#978)

* Replace supervisord xmlrpc library
* Use `github.com/mattn/go-xmlrpc` that doesn't leak goroutines.
* Fix uptime metric

* Use Prometheus best practices for uptime metric.
  * Use "start time" rather than "uptime".
  * Don't emit a start time if the process is down.
* Add changelog entry.
* Add example compatibility rules.

Signed-off-by: Ben Kochie <superq@gmail.com>

vendor/github.com/mattn/go-xmlrpc/README.md

@@ -0,0 +1,48 @@
+# go-xmlrpc
+
+xmlrpc interface for go
+
+## Usage
+
+```go
+package main
+
+import (
+	"github.com/mattn/go-xmlrpc"
+	"fmt"
+	"log"
+)
+
+func main() {
+	res, e := xmlrpc.Call(
+		"http://your-blog.example.com/xmlrpc.php",
+		"metaWeblog.getRecentPosts",
+		"blog-id",
+		"user-id",
+		"password",
+		10)
+	if e != nil {
+		log.Fatal(e)
+	}
+	for _, p := range res.(xmlrpc.Array) {
+		for k, v := range p.(xmlrpc.Struct) {
+			fmt.Printf("%s=%v\n", k, v)
+		}
+		fmt.Println()
+	}
+}
+```
+
+## Installation
+
+```
+$ go get github.com/mattn/go-xmlrpc
+```
+
+## License
+
+MIT
+
+## Author
+
+Yasuhiro Matsumoto (a.k.a. mattn)