Add threading spec.

Thanks to @dturnbull for providing these cases in
#98.

spec/threading_spec.rb

@@ -0,0 +1,24 @@
+require 'spec_helper'
+
+describe "using v8 from multiple threads" do
+
+  it "creates contexts from within threads" do
+    10.times.collect do
+      Thread.new do
+        V8::Context.new
+      end
+    end.each {|t| t.join}
+    V8::Context.new
+  end
+
+  it "executes codes on multiple threads simultaneously" do
+    5.times.collect{V8::Context.new}.collect do |ctx|
+      Thread.new do
+        ctx['x'] = 99
+        while ctx['x'] > 0
+          ctx.eval 'for (i=10000;i;i--){};--x'
+        end
+      end
+    end.each {|t| t.join}
+  end
+end