deprecate Promise.prototype.spawn
Simplify nextTick bug workaround

Fixes #129
Remove describe.only as it blocks other tests to run

I used it to filter out other tests to speed up the development, but forgot to remove before i had sent previous PR to fix the issue with process.nextTick. My bad.
Implement addYieldHandler
Merge branch 'master' of github.com:petkaantonov/bluebird