Support for multiple digest algorithms in signature verification

 1. Detect used digest algorithms for node digesting and signature verification
 2. Add support for SHA256 and GOST R 34.11-94 algorithms out of the box.
 3. Add possibility to add custom digest algorithms on the fly.
 4. Test for SHA256 signature and node digests.
 5. Test for GOST R 34.11-94 node digests (It's hard to test GOST signing as it requires patched Ruby, see: https://bugs.ruby-lang.org/issues/9830 )
Separately access GOST R 34.11-94 digest algorithm in JRuby
Merge pull request #13 from Envek/more_verification_algorithms

Support for multiple digest algorithms in signature verification