Add download script to obtain PowerShell release

This requires that $GITHUB_TOKEN be set. Travis sets this as a secure
environment variable; a user can provider their own.

download.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+
+[[ -n $GITHUB_TOKEN ]] || { echo >&2 "GITHUB_TOKEN variable is undefined, please provide token"; exit 1; }
+
+# Authorizes with read-only access to GitHub API
+# Retrieves URL of v0.2.0 release asset
+# Downloads asset to powershell.deb
+curl -s -i \
+     -H "Authorization: token $GITHUB_TOKEN " \
+     -H 'Accept: application/octet-stream' \
+     'https://api.github.com/repos/PowerShell/PowerShell/releases/assets/1398546' |
+    grep location |
+    sed 's/location: //g' |
+    wget -i - -O powershell.deb