More IE7 fixes: atRule and unit test

Hi,

In IE7, the ```Modernizr.prefixed("@import")``` test was throwing an exception in following lines:

```javascript
if (rule in cssrule)
...
if (thisRule in cssrule)
```

This is because cssrule is undefined. I added a check to handle that [corner] case.

Also, *caniuse* test suite was dying because of failure to parse certain results in JSONP. Specifically, those which have form 'a #*i*' or 'y #*i*', where *i* is numeric. I would think this refers to some notes regarding support of particular property.   I've added more relaxed comparison with 'a' and 'y' to allow test suite to at least complete execution normally in IE7.

Would be happy if you guys could consider merging this.

Thanks,
Vlad.

Test 49 (drag and drop) for IE7.

With the patch above, with addition of *caniuse* reference value to the test message, I get this:
```Caniuse reported partial support for dragndrop a #1. So.. Modernizr's true is good enough...```

As for 'y'-related chunk of the patch - it was not the case for IE, but in JSONP output from *caniuse* I've seen such a values, like ```y #1``` - you can call http://caniuse.com/jsonp.php?callback=caniusecb and simply search the output. I consequently assume that those may cause failure with different browser versions - apparently I cannot test all of them, but I looking on the code above I bet it will fail similarly to ```a #<...>``` value.