The compare_size cache gets wiped out with `grunt clean`

`.grunt/` is the directory that is suggested for storing task related
files, and this file is not useful if it gets wiped out often. So I'm
moving the compare_size cache into `.grunt/` so we keep it around
longer.
Merge pull request #124 from zpao/keep-compare-size-cache

The compare_size cache gets wiped out with `grunt clean`