Add a special case for empty `/*` comments

Looks like they didn't fall into quite the right position!

Closes #57
diff --git a/src/strnom.rs b/src/strnom.rs
index 1bf9f23..33964f4 100644
--- a/src/strnom.rs
+++ b/src/strnom.rs
@@ -80,6 +80,9 @@
                     continue;
                 }
                 break;
+            } else if s.starts_with("/**/") {
+                i += 4;
+                continue
             } else if s.starts_with("/*") && (!s.starts_with("/**") || s.starts_with("/***")) &&
                       !s.starts_with("/*!") {
                 let (_, com) = block_comment(s)?;