resolves #366 enhance Rakefile to properly handle release

You can now release the gem using:

 $ rake release

It will remind you to update the version if it contains ".dev".

- updates the date in the gemspec
- creates the tag in git using pattern v%version%
- prevents release if version contains .dev
- pushes the repo to origin/master
- builds and pushes the package to rubygems.org
- updated the description of project
- added missing authors
- use git ls-tree to create package file lists