crypto: add sync methods, deprecate optional callbacks

See https://github.com/iojs/io.js/issues/5

- would you like me to add/change tests?
- didn't bother disallowing callbacks in the `sync` version. after all this is about the public API. is that something you guys want?
- want doc updates?
- only changed `.randomBytes()` and `.pseudoRandomBytes()`. were there others?