src: accept passphrase when crypto signing with private key

Previous behaviour was to drop to an openssl prompt
("Enter PEM pass phrase:") when supplying a private key with a
passphrase. This change adds a fourth, optional, paramter that
will be used as the passphrase.
To include this parameter in a backwards compatible way it was
necessary to expose the previously undocumented (and unexposed)
feature of being able to explitly setting the output encoding.
src: Remove unused refs to node_object_wrap.h

Turns out that we don't use node_object_wrap.h any more in core,
and, with v8 3.21, it's breaking our Windows build. Removing refs
to it everywhere (and adding node.h in one case where it was the
only way node.h was being included), we have restored the Windows
build.