Add ARM target for binaries

```
$ make build-cross dist VERSION=v2.2.0
CGO_ENABLED=0 gox -output="_dist/{{.OS}}-{{.Arch}}/{{.Dir}}" -osarch='darwin/amd64 linux/amd64 linux/386 windows/amd64 linux/arm'  -tags '' -ldflags ' -X k8s.io/helm/pkg/version
.Version=v2.1.0 -X k8s.io/helm/pkg/version.GitCommit=b3d812b3462e5ac8192f656c7098b1b54f29ffa3 -X k8s.io/helm/pkg/version.GitTreeState=dirty -extldflags "-static"' k8s.io/helm/cm
d/helm
Number of parallel builds: 3

-->    darwin/amd64: k8s.io/helm/cmd/helm
-->     linux/amd64: k8s.io/helm/cmd/helm
-->   windows/amd64: k8s.io/helm/cmd/helm
-->       linux/386: k8s.io/helm/cmd/helm
-->       linux/arm: k8s.io/helm/cmd/helm

```

```
$ ls -a _dist/
.   darwin-amd64                     helm-v2.2.0-linux-386.tar.gz    helm-v2.2.0-linux-arm.tar.gz      linux-386    linux-arm
..  helm-v2.2.0-darwin-amd64.tar.gz  helm-v2.2.0-linux-amd64.tar.gz  helm-v2.2.0-windows-amd64.tar.gz  linux-amd64  windows-amd64
```

Makefile

@@ -1,7 +1,7 @@
 DOCKER_REGISTRY ?= gcr.io
 IMAGE_PREFIX    ?= kubernetes-helm
 SHORT_NAME      ?= tiller
-TARGETS         = darwin/amd64 linux/amd64 linux/386 windows/amd64
+TARGETS         = darwin/amd64 linux/amd64 linux/386 linux/arm windows/amd64
 DIST_DIRS       = find * -type d -exec
 
 # go option