fix(gruntfile): watch subdirectories at any level without killing cpu

I know this is a largely discussed topic (here, for example: #442), but I didn't find someone who tried this solution: watching all subdirectories (at any level) in the ```app``` directory, except for ```bower_components```.

```bower_components``` is huge (thousands of files, in my new empty scaffolded project it contains **477** files) and I guess it's the one who increases the cpu usage.

Key lines are 69 and 70 in my commit:

    '<%%= yeoman.app %>/*.html'
    '<%%= yeoman.app %>/views/**/*.html'

instead of
 
    '<%%= yeoman.app %>/**/*.html'

However, I'm not an expert and I wouldn't be surprised if I were wrong. :smiley: