moving Contributing from README to its own file
Contributing: add note about making changes to the release file
add npm devDependency: jshint
Gemfile: use HTTPS Rubygems source
rework Rakefile

 * use local version of jshint so all developers are on the same version
 * run jshint and tests on a version compiled to tmp/ instead of the
   public distribution so that developers don't create a distribution
   just to run jshint/tests
 * create a new dist task that copies the file from tmp/ to timecop.js
dist file no longer has a version number
Rakefile: use Pathname for happier joins