Merge pull request #2 from slimlv/master

DKIM hash/signatures verification works now
Fixed defined check

defined() has string $name as argument, removed constant()
Use correct signature mode with Crypt_RSA

Default signature mode for Crypt_RSA is CRYPT_RSA_SIGNATURE_PSS (in phpseclib v0.3.6), but here we need CRYPT_RSA_SIGNATURE_PKCS1 to verify signature correctly
Merge pull request #3 from pimlie/master

Fixed signature verification with Crypt_RSA