Add std::exception as a base clase off std::runtime_error and provide unit test for it.

unittests/eval_catch_exception_test.cpp

@@ -0,0 +1,21 @@
+// Tests to make sure that the order in which function dispatches occur is correct
+
+#include <chaiscript/chaiscript.hpp>
+
+
+int main()
+{
+  chaiscript::ChaiScript chai;
+  
+  try {
+    chai.eval("throw(runtime_error(\"error\"));");
+  } catch (const chaiscript::Boxed_Value &bv) {
+    const std::exception &e = chaiscript::boxed_cast<const std::exception &>(bv);
+    if (e.what() == std::string("error"))
+    {
+      return EXIT_SUCCESS;
+    }
+  }
+
+  return EXIT_FAILURE;
+}