Include more files in the gem

Adding the following files:
  - spec/ to be able to run the specs from the installed gem
  - COPYING, GPLv2, GPLv3 and LICENSE, for legal reasons
  - Rakefile and Gemfile to be able to use rake targets to run the tests

The README.md file is at the moment almost empty, so there is probably
no reason to include it yet.
Inclusion of Rakefile and Gemfile are may be optional, but can be useful.
Merge pull request #3 from prawnpdf/gemspec_files

Include more files in the gem