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.
add bitcore.KeyModule back as deprecated

This will allow bitcore.KeyModule to work with existing code, but it prints out
a message that it is deprecated.
Merge pull request #152 from ryanxcharles/feature/fix-key-interface

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