Make gem self versioning

nullgem.gemspec

@@ -1,7 +1,18 @@
 Gem::Specification.new do |gem|
   gem.name          = "nullgem"
-  gem.version       = '0.0.0'
-  gem.date          = '2001-01-01'
+  gem.version     = begin
+    head_sha          = %x{git rev-parse HEAD}.strip
+    tag_version       = %x{git name-rev --tags --name-only #{head_sha}}.strip
+    untracked_files   = (%x{git ls-files --others --exclude-standard}.strip != '')
+    _diff_status_only =  %x{git diff --quiet --exit-code HEAD}
+    head_diff = ($?.exitstatus != 0)
+
+    version = (tag_version =~ /^\d+\.\d+\.\d+$/) ? tag_version : "0.0.0.#{head_sha[0..7]}"
+    dev_tag = (untracked_files || head_diff) ? '-dev' : ''
+
+    version + dev_tag
+  end
+  gem.date          = '2012-03-04'
   gem.authors       = ["Michael Toy"]
   gem.email         = ["[email protected]"]
   gem.description   = %q{A gem which does nothing, for testing}