Several minor changes to the development process

* Add compare_size for better file size diffs, FYI this is the same used
by jQuery;
* Updating uglify removes reports by default, let's delegate a better
diff for compare_size task instead;
* Add grunt watch to develop without having to type grunt everytime
(need to add to the contributing docs);
* Separate cli task from grunt default, so we can enforce minify and
compare_size checks for local development;
* Add gzip compression for compare_size so we can compare gzipped
content too;