Add project metadata to the gemspec

As per https://guides.rubygems.org/specification-reference/#metadata,
add metadata to the gemspec file. This'll allow people to more easily
access the source code, raise issues and read the changelog. These
`bug_tracker_uri`, `changelog_uri`, `documentation_uri`, and
`source_code_uri` links will appear on the rubygems page at
https://rubygems.org/gems/crass and be available via the rubygems API
after the next release.

crass.gemspec

@@ -11,6 +11,13 @@ Gem::Specification.new do |s|
   s.homepage    = 'https://github.com/rgrove/crass/'
   s.license     = 'MIT'
 
+  s.metadata = {
+    'bug_tracker_uri'   => 'https://github.com/rgrove/crass/issues',
+    'changelog_uri'     => "https://github.com/rgrove/crass/blob/v#{s.version}/HISTORY.md",
+    'documentation_uri' => "https://www.rubydoc.info/gems/crass/#{s.version}",
+    'source_code_uri'   => "https://github.com/rgrove/crass/tree/v#{s.version}",
+  }
+
   s.platform = Gem::Platform::RUBY
   s.required_ruby_version = Gem::Requirement.new('>= 1.9.2')