actualized rakefile and gemspec a little bit

Some improvements over current rakefile and packaging:
- prevents "ERROR: 'rake/gempackagetask' is obsolete and no longer supported. Use 'rubygems/packagetask' instead." from happening, this leads to halting all rake tasks
- bundler support and Gemfile
- cleaned up rakefile
- packaging tasks delegated to Bundler::GemHelper
- version to packaging task not hardcoded anymore
- rake test
- removed useless require statements

Motivation and notes on packaging: http://yehudakatz.com/2010/04/02/using-gemspecs-as-intended/