refactor batch add out of controller
do batch_add in the background
respond with status 202 because request is not resolved no response
remove cucumber dependency
User Oj gem for serializing Hash to JSON.

For this I had to change ContactSerializer.serialize's Hash
Returned hash now has Strings as keys instead of Symbols
Revert "User Oj gem for serializing Hash to JSON."

This reverts commit 579e7632af51085fadbe0b6318107ae660b3f28a.
serialize id as a string, not BSON
stringify keys

For Oj dump to be backward compatible
Revert "Revert "User Oj gem for serializing Hash to JSON.""

This reverts commit c503983ba21dd36901fff3f1b2cefd090a71198c.
Merge branch 'faster-serializer'
Configurations for staging deploy on heroku