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
Fix for bug in MailMessage reported on forum
(http://pocoproject.org/forum/viewtopic.php?f=12&t=6106&sid=9283682f915ea148d86813ea140cd3c7).
MailMessage was not handling encoding when no Content-Transfer-Encodign
header was not present for MultiPart messages, resulting on an
Exception. His fix checks if the header is present before getting is
value, and defaults to 7bit encoding as in
http://www.w3.org/Protocols/rfc1341/5_Content-Transfer-Encoding.html
gh #361

"hpux" or "_hpux" doesn't exist on HPUX platform #361
Parse JSON as structure #365
Merge pull request #386 from cristiantm/fix-mailmessage

Fix for bug in MailMessage without content-transfer-encoding header
style fix
Merge pull request #384 from cristiantm/improve-rsadigest

New hash algorithms support for RSADigestEngine
style fix
Merge remote-tracking branch 'upstream/develop' into develop