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.