Rule: No conditional expression (Closes #307)

Closes #307 I'm not 100% I caught all use cases that matter and that JSHint W030 rule is supposed to catch. W030 rules is also supposed to catch the following case:
```
var a = 0;
a;
```
But I have no idea how to catch something like that. Let me know if you don't want to merge it as it is now.