[idan] Improve Bidi Rules implementation

* As the first paragraph of The Bidi Rules section explains, the rules
need to be ignored if there are no Bidi labels present in the domain
name. So, add `is_bidi_domain` evaluation to `processing()`, and pass it
down to `passes_bidi()` to act on.

* Add unit tests for the bidi rules, making it faster and easier to
maintain the feature.

Author: behnam

idna/Cargo.toml

@@ -20,5 +20,5 @@ rustc-serialize = "0.3"
 
 [dependencies]
 unicode-bidi = "0.3"
-unicode-normalization = "0.1.3"
+unicode-normalization = "0.1.5"
 matches = "0.1"