add rake tasks to release the gem

so that you can do:

± rake -T
rake build    # Build git-issues-0.1.3.gem into the pkg directory
rake install  # Build and install git-issues-0.1.3.gem into system gems
rake release  # Create tag v0.1.3 and build and push git-issues-0.1.3.gem to Rubygems
rake test     # Run tests

not tested in this repo because you can not delete a ruby gem from rubygems.org