Add codecoverage with 'meteor test' command

[allonzy]

I try to set up a CI with meteor and there is no code-coverage feature on meteor, that's a pity.I don't need to explain why code-coverage feature is great :stuck_out_tongue:. I've seen an implementation of this feature by serut, but it is not very easy to set up, and is not sync with the latest istanbul.js release. I'll recommand to add call to istanbul.js in the testing command pipeline I can't implement this feature myself because i'm poorly experimented with meteor and node.js in general.