update gemspec to a modern version

- update gemspec to a modern version
  by removing depenency on older rubygems specs
  and all gems dependencies have versions
- add gem_tasks to Rakefile
- add file to track version
- add MIT License header to LICENSE
- update gitignore
- add installation instructions to README
Merge pull request #2 from deepak/chore/update-gemspec

update gemspec to a modern version