add WORKSPACE file

This is necessary in order to build with Bazel.

Tested on Linux with the following command line:

$ bazel build //:all
diff --git a/WORKSPACE b/WORKSPACE
new file mode 100644
index 0000000..52106e7
--- /dev/null
+++ b/WORKSPACE
@@ -0,0 +1 @@
+# Bazel (http://bazel.io/) WORKSPACE file for double-conversion.