Cross compile and README update

Makefile

@@ -4,7 +4,7 @@
 VERSION = $(shell git describe --always --dirty)
 TIMESTAMP = $(shell git show -s --format=%ct)
 
-default: build
+default: build_darwin
 
 setup: 
 	go get -u github.com/jteeuwen/go-bindata/...
@@ -12,13 +12,34 @@ setup:
 resources:
 	go-bindata -pkg resources -o internal/resources/resources.go resources/...
 
-build: resources
-	go build -o ./gopow *.go
+build_darwin: resources
+	GOOS=darwin GOARCH=amd64 go build -a -o ./build/gopow *.go
+	zip ./build/gopow_darwin64.zip ./build/gopow
+
+build_linux: resources
+	GOOS=linux GOARCH=amd64 go build -a -o ./build/gopow *.go
+	zip ./build/gopow_linux64.zip ./build/gopow
+
+build_arm5: resources
+	GOOS=linux GOARM=5 GOARCH=arm go build -a -o ./build/gopow *.go
+	zip ./build/gopow_linux_arm5.zip ./build/gopow
+
+build_arm7: resources
+	GOOS=linux GOARM=7 GOARCH=arm go build -a -o ./build/gopow *.go
+	zip ./build/gopow_linux_arm7.zip ./build/gopow
+
+build_win: resources
+	GOOS=windows GOARCH=amd64 go build -a -o ./build/gopow.exe *.go
+	zip ./build/gopow_win64.zip ./build/gopow.exe
+
+all: build_darwin build_linux build_arm5 build_arm7 build_win
+	rm ./build/gopow
+	rm ./build/gopow.exe
 
 lint:
 	golint .
 
 clean:
-	rm -f gopow
+	rm -f build
 	rm -rf internal/resources