update interface, bitcore.KeyModule.Key -> bitcore.Key

It's annoying and easy to forget to type in bitcore.KeyModule.Key. I have
updated this so that now you can just do bitcore.Key. Tests pass in node and
the browser. This is a backwards-incompatible change so all software that
depends on the old style key generation will need to be updated.

This addresses this issue: https://github.com/bitpay/bitcore/issues/146