Adding missing signing_key to gemspec

per http://developer.zendesk.com/blog/2013/02/03/signing-gems/
    and https://github.com/sferik/twitter/blob/master/twitter.gemspec

1. Create self-signed cert

 cd ~/.ssh
 gem cert --build your@email.com
 chmod 600 gem-p*

2. Configure gem cert (I'm calling mine bf4)

 mkdir certs
 cp ~/.ssh/gem-public_cert.pem certs/bf4.pem
 - commit the pem
 - Add to gemspec
 s.cert_chain  = ['certs/bf4.pem']
 s.signing_key = File.expand_path("~/.ssh/gem-private_key.pem") if $0 =~ /gem\z/

3. Add your own cert, just like anyone else

 gem cert --add certs/bf4.pem

4. Build gem and test that you can install it

 gem build metric_fu.gemspec
 gem install metric_fu-4.5.1.gem -P HighSecurity

metric_fu.gemspec

@@ -13,6 +13,7 @@ Gem::Specification.new do |s|
 
   # used with gem i metric_fu -P HighSecurity
   s.cert_chain  = ['certs/bf4.pem']
+  s.signing_key = File.expand_path("~/.ssh/gem-private_key.pem") if $0 =~ /gem\z/
 
   s.rubyforge_project           = 'metric_fu'
   s.license                     = 'MIT'