Download and install packages for Ubuntu and OS X

This both downloads the v0.3.0 release for the current platform and now
also installs it (requiring `sudo`). Should probably rename.

download.sh

@@ -2,13 +2,39 @@
 
 [[ -n $GITHUB_TOKEN ]] || { echo >&2 "GITHUB_TOKEN variable is undefined, please provide token"; exit 1; }
 
+# Set OS specific asset ID and package name
+case "$OSTYPE" in
+    linux*)
+        asset='1536045'
+        package='powershell_0.3.0-1_amd64.deb'
+        ;;
+    darwin*)
+        asset='1536063'
+        package='powershell-0.3.0.pkg'
+        ;;
+    *)
+        exit 2 >&2 "$OSTYPE not supported!"
+        ;;
+esac
+
 # Authorizes with read-only access to GitHub API
 # Retrieves URL of v0.3.0 release asset
-# Downloads asset to powershell.deb
 curl -s -i \
      -H "Authorization: token $GITHUB_TOKEN " \
      -H 'Accept: application/octet-stream' \
-     'https://api.github.com/repos/PowerShell/PowerShell/releases/assets/1536045' |
+     "https://api.github.com/repos/PowerShell/PowerShell/releases/assets/$asset" |
     grep location |
     sed 's/location: //g' |
-    wget -i - -O powershell.deb
+    wget -i - -O $package
+
+# Installs PowerShell package
+case "$OSTYPE" in
+    linux*)
+        sudo dpkg -i ./$package
+        ;;
+    darwin*)
+        sudo installer -pkg ./$package -target /
+        ;;
+esac
+
+echo "Congratulations! PowerShell is now installed."