Add homepage, repository and bugs information to package.json

(Note: This is fixing the same problem that [htmlmin also had](https://github.com/gruntjs/grunt-contrib-htmlmin/pull/57). These two contrib plugins seemed to be different than the rest.)

I noticed that the grunt-contrib README list of projects had a [broken/missing link for ```grunt-contrib-imagemin```](https://github.com/gruntjs/grunt-contrib#grunt-contrib-imagemin-v050--)

I think I've tracked it down to this project lacking the homepage field in the ```package.json```. It also was missing a couple other fields that the other grunt-contrib plugins have.

This pull request adds in those missing fields.