Add cross-compilation Makefile targets and tar-based releases.

Revamp the build system to be more inline with other Prometheus exporters.
Notably add Darwin and Windows build targets, and add support for releases
using tar files.

tools/vendor/github.com/GoASTScanner/gas/Dockerfile

@@ -0,0 +1,23 @@
+# Docker version must be 17.05 or higher to allow multistage build
+# See build and run instructions in README.md
+
+# Builds Gas for utilization
+FROM golang:1.8.1-alpine as builder
+ENV workspace /go/src/github.com/GoASTScanner/gas
+ENV GOPATH /go
+COPY . $workspace
+WORKDIR $workspace
+
+RUN go vet $(go list ./... | grep -v /vendor/)
+RUN CGO_ENABLED=0 go build -o gas .
+
+########################################################
+
+# Runs Gas on all Go files in the current directory when 
+# 'docker run' command in README is given
+FROM alpine:3.6
+
+COPY --from=builder /go/src/github.com/GoASTScanner/gas/gas /
+
+# Mounted directory should be placed into the workdir
+CMD /gas $(find . -path ./vendor -prune -o -type f -name "*.go")