Rename to atmosphere convention

My previous commit actually broke `meteor add featureFlag` :\ 
I suggest to rename package to feature-flag to 
- Follow guidelines from atmosphere - https://atmospherejs.com/docs/publishing
- Make it easier to migrate to meteor 0.9.0. As meteorite is gonna be deprecated soon, looks like the new meteor wants a package name to match the repo name. Which will be just matter of removal meteor prefix from this one.

If you are ok with this one and https://github.com/recursivefaults/meteor-feature-flag/pull/4, can you release a new version on atmosphere after this is merged, please?