How can I watch all addresses under HD wallet?
Full disclosure: I'm BlockCypher's Developer Advocate.

We have a series of HD Wallet Endpoints that we recently released that might suit your needs; you can read more about it here:

http://dev.blockcypher.com/#hierarchical-deterministic-hd-wallets

You can use it in tandem with our WebHooks API for notifications whenever an address associated with an HD wallet is involved in a transaction.

If you want to use something locally instead of our API, I'm a fan of btcd and their associated btcwallet, which supports HD addresses.