Update for dart-1.1 breaking changes

    I couldn't get this nice looking library on pub repository compiled with the latest Dart 1.1. So this little commit comes up.
    
    * Changed a dependent "browser" version to ">=0.9.1"
    * Replaced deprecated calls like "json.stringfy" with "convert.something"
    * Implemented hashCode() function for "Point2" class.

I'm not really sure I could just change "browser" version to 0.9.1. But "'>=0.3.5+1 <0.3.6'" was too restrictive to incorporate with other libraries.