Build script for Windows. (#3481)

A re-implementation of build.sh for Windows ala PowerShell.  This is as
close to one-to-one that I could make it.

.gitignore

@@ -8,6 +8,7 @@
 .vagrant
 .idea
 test/.env
+*~
 
 website/.bundle
 website/vendor

scripts/build.ps1

+<#
+    .Synopsis
+    Build script for Packer.
+
+    .Description
+    Build script for Packer for all supported platforms and architectures.
+    By default the following OSs and architectures are targeted.
+
+    OS:
+     * linux
+     * darwin
+     * windows
+     * freebsd
+     * openbsd
+
+    Architecture:
+     * 386
+     * amd64
+     * arm
+
+    If the environment variable PACKER_DEV is defined, then the OS and
+    architecture of the go binary in the path is used.
+
+    The built binary is stamped with the current version number of Packer,
+    the latest git commit, and +CHANGES if there are any outstanding
+    changes in the current repository, e.g.
+
+      Packer v0.10.1.dev (3c736322ba3a5fcb3a4e92394011a2e56f396da6+CHANGES)
+
+    The build artifacts for the current OS and architecture are copied to
+    bin and $GOPATH\bin.
+
+    .Example
+    .\scripts\build.ps1
+#>
+
+# This script builds the application from source for multiple platforms.
+
+# Get the parent directory of where this script is.
+$DIR = [System.IO.Path]::GetDirectoryName($PSScriptRoot)
+
+# Change into that directory
+Push-Location $DIR | Out-Null
+
+# Get the git commit
+$GIT_COMMIT = $(git.exe rev-parse HEAD)
+git.exe status --porcelain | Out-Null
+if ($LastExitCode -eq 0) {
+    $GIT_DIRTY = "+CHANGES"
+} 
+
+# If its dev mode, only build for ourself
+if (Test-Path env:PACKER_DEV) {
+    $XC_OS=$(go.exe env GOOS)
+    $XC_ARCH=$(go.exe env GOARCH)
+}
+elseif (-not (Test-Path env:XC_ARCH)) {
+    $XC_ARCH="386 amd64 arm"
+    $XC_OS="linux darwin windows freebsd openbsd"
+}
+
+# Delete the old dir
+echo "==> Removing old directory..."
+Remove-Item -Recurse -ErrorAction Ignore -Force "bin\"
+Remove-Item -Recurse -ErrorAction Ignore -Force "pkg\"
+New-Item -Type Directory -Name bin | Out-Null
+
+# Delete the old dir
+echo "==> Building..."
+gox.exe `
+  -os="${XC_OS}" `
+  -arch="${XC_ARCH}" `
+  -ldflags "-X github.com/mitchellh/packer/version.GitCommit ${GIT_COMMIT}${GIT_DIRTY}" `
+  -output "pkg/{{.OS}}_{{.Arch}}/packer" `
+  .
+
+if ($LastExitCode -ne 0) {
+    exit 1
+}
+
+# Move all the compiled things to the $GOPATH/bin
+$GOPATH=$(go.exe env GOPATH)
+
+# Copy our OS/Arch to the bin/ directory
+echo "==> Copying binaries for this platform..."
+Get-ChildItem ".\pkg\$(go env GOOS)_$(go env GOARCH)\" `
+  |? { !($_.PSIsContainer) } `
+  |% {
+      Copy-Item $_.FullName "bin\"
+      Copy-Item $_.FullName "${GOPATH}\bin\"
+  }
+
+# Done!
+echo "`r`n==> Results:"
+Get-ChildItem bin\