Ethkey - extended keys (#4377)

* data structures initial

* hard derivation

* tabs, docs

* more docs

* soft private derivation

* public derivation

* finalize api, fix warnings

* use simple new()

* keypair api

* bump byteorder

* doc tweaks

* remove heavyness from tests

* added test vector infrastructure and examples

* initialization from seed to key pair

* add comment about panic