Improve RSADigestEngine, using Poco::Crypto::DigestEngine to calculate
the hash before signing. That way we can then support RSA signatures
with any hash supported by OpenSSL, not only MD5 and SHA1. It was also
required a new method on DigestEngine to obtain the NID (numeric ID) of
the algorithm, to be used on the OpenSSL RSA_sign call.
Using the same variable name as in DigestEngine
Fixing #includes. pem.h and evp.h are not needed, only rsa.h
Also removing include of rsa.h that is not needed on the header file
Merge pull request #384 from cristiantm/improve-rsadigest

New hash algorithms support for RSADigestEngine