Add support for version numbers in the Rakefile

* version number is stored in version.yaml
  - work in progress is marked with version number that ends with '-snapshot'
* all compiled files are stored in the './build/' directory without version numbers
* :package task creates a tarball in the build directory
  - if version number contains '-snapshot', this substring is replaced with sha of the head
  - tarball contains version number in the filename
  - all js files contain version number in the filename
* .gitignore was updated to reflect all these changes
* the .map file is not created by the closure compiler any more