Add compatibility with @page css declaration (twitter bootstrap)

The 0.0.6 release of ```css-condense``` is not compatible with the ```@page``` css declaration (used by twitter bootstrap css, for instance) 
This pull request:
 - narrow dependencies
 - add test for ```@page```

css-stringify>=1.3.0 produce compiler error
css-parse>=1.2.0 doesn't support @page declarations

Fixes https://github.com/rstacruz/css-condense/issues/5