crypto: Native SPKAC support

Implements new class 'Certificate' within crypto object
with (as of this commit) three new functions for working
with SPKAC's (signed public key & challenge).

bool Certificate::verifySpkac(buffer)
buffer Certificate::exportPublicKey(buffer)
buffer Certificate::exportChallenge(buffer)
crypto: Moved away from conversion to a slow buffer
crypto: Test case for SPKAC tests
crypto: Finished test case(s)
crypto: Suggested fixes
crypto: Additional suggested fixes
crypto: Double line between methods
crypto: Added node_isolate to boolean return arguments