Add script to update version of eris.

- Globally replaces old version for new throughout the codebase,
  commits the change, then creates a tag for the new version at the head.
- This is instead of magic code within setup.py that tries to pull
  the version from git at runtime. Also git isn't always available.

packaging/update-version

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+
+set -e
+
+
+# Script only runs at the codebase root.
+[ $(basename $PWD) == "eris" ]
+[ -e README.md ]
+
+
+NEW_VERSION=$(date "+%Y.%m.%d")
+CURRENT_VERSION=$(git describe --tags --abbrev=0)
+if [ $NEW_VERSION == $CURRENT_VERSION ]; then
+    git tag --delete $CURRENT_VERSION
+else
+    git grep -l $CURRENT_VERSION | xargs sed -i "s/$CURRENT_VERSION/$NEW_VERSION/g"
+    git commit --all --message="Update version from $CURRENT_VERSION to $NEW_VERSION."
+fi
+git tag $NEW_VERSION