Native SPKAC support

crypto: Implements new class 'Certificate' within crypto object
with (as of this commit) three new functions for working
with SPKAC's. [Issue #5546](https://github.com/joyent/node/issues/5546) as discussed with @bnoordhuis

```
bool Certificate::verifySpkac(buffer)
buffer Certificate::exportPublicKey(buffer)
buffer Certificate::exportChallenge(buffer)
```

Also includes tests