Add build-phar task to build file
Split release process into two tasks: Preparing and creating.

This allows for manually adjusting changelog and tag message.

Add phar building to release preparation.
Release 2.1.0
Creating a GitHub release and upload resulting phar file when releasing.

- Added required phing-github tasks and allow dev requirements as this is
required by PhingGitHub but prefer stable releases.
- Restructured build.xml.
- Updated README with path to phar file and latest release
- Ignore potential build leftovers TAG_MESSAGE