Support browser environment too.

I was working on some stuff for [graft](http://github.com/graftjs/graft), which involves piping streams into the browser,
but I found that this module wasn't logging because it sends a passhtrough stream when process.env.DEBUG is empty.

Debug actually supports the browser too, triggered by a special helper function that toggles something in localStorage,
so I exposed this functionality here too.

I used the same checking code as debug does (the try/catch) to avoid inconsistencies. also updated the readme.