Table custom errors until a later date. Remove need for devDependencies ...

- Fixes installation when using `npm install bitpay/bitcore#v0.8` and requiring in `package.json` #650 by removing need to run devDependencies
- Stack traces are correctly reported for errors #646 by tabling custom errors, we can revisit later with #651
- When instantiatiating an HDPublicKey with an extended privatekey will return the associated HDPublicKey 
This already works all right, and there's a test for it here:
https://github.com/bitpay/bitcore/blob/v0.8/test/hdpublickey.js#L58