Add support for default PHP padding : "OPENSSL_PKCS1_PADDING"

Would be awesome as i'm stuck on this on a project (using PHP on the server side).
PHP : http://php.net/manual/en/function.openssl-public-encrypt.php

Tried to manually replace occurrences of OPENSSL_PKCS1_PADDING with it.

But i get a build error `node-waf configure build` :

```
../node_rsa.cc:310: error: ‘OPENSSL_PKCS1_PADDING’ was not declared in this scope
```
