Refactor repository layout and convert build system to Mage.

This commit implements a massive refactor of the repository, and
moves the build system over to use Mage (magefile.org) which should
allow seamless building across multiple platforms.

tools/vendor/github.com/mattn/goveralls/README.md

@@ -137,6 +137,32 @@ test:
 
 For more information, See https://coveralls.zendesk.com/hc/en-us/articles/201342809-Go
 
+## Sempahore
+
+Store your Coveralls API token in `Environment Variables`:
+
+```
+COVERALLS_TOKEN=your_token_goes_here
+```
+
+More instructions on how to do this can be found in the [Semahore documentation](https://semaphoreci.com/docs/exporting-environment-variables.html).
+
+Replace the `go test` line in your `Commands` with these lines:
+
+```
+$ go get github.com/mattn/goveralls
+$ goveralls -service semaphore
+```
+
+`goveralls` automatically use the environment variable `COVERALLS_TOKEN` as the
+default value for `-repotoken`.
+
+You can use the `-v` flag to see verbose output from the test suite:
+
+```
+$ goveralls -v -service semaphore
+```
+
 # Authors
 
 * Yasuhiro Matsumoto (a.k.a. mattn)

tools/vendor/github.com/mattn/goveralls/goveralls.go

@@ -54,6 +54,7 @@ var (
 	service    = flag.String("service", "travis-ci", "The CI service or other environment in which the test suite was run. ")
 	shallow    = flag.Bool("shallow", false, "Shallow coveralls internal server errors")
 	ignore     = flag.String("ignore", "", "Comma separated files to ignore")
+	show       = flag.Bool("show", false, "Show which package is being tested")
 )
 
 // usage supplants package flag's Usage variable
@@ -149,6 +150,9 @@ func getCoverage() ([]*SourceFile, error) {
 		args = append(args, line)
 		cmd.Args = args
 
+		if *show {
+			fmt.Println("goveralls:", line)
+		}
 		err = cmd.Run()
 		if err != nil {
 			return nil, fmt.Errorf("%v: %v", err, outBuf.String())
@@ -234,6 +238,8 @@ func process() error {
 		jobId = circleCiJobId
 	} else if appveyorJobId := os.Getenv("APPVEYOR_JOB_ID"); appveyorJobId != "" {
 		jobId = appveyorJobId
+	} else if semaphoreJobId := os.Getenv("SEMPAHORE_BUILD_NUMBER"); semaphoreJobId != "" {
+		jobId = semaphoreJobId
 	}
 
 	if *repotoken == "" {
@@ -250,6 +256,8 @@ func process() error {
 		pullRequest = regexp.MustCompile(`[0-9]+$`).FindString(prURL)
 	} else if prNumber := os.Getenv("APPVEYOR_PULL_REQUEST_NUMBER"); prNumber != "" {
 		pullRequest = prNumber
+	} else if prNumber := os.Getenv("PULL_REQUEST_NUMBER"); prNumber != "" {
+		pullRequest = prNumber
 	}
 
 	sourceFiles, err := getCoverage()