Allow to sign with another digest algorithms (for both nodes digesting and signing)

1. Allow to specify digest algorithms for node digesting and signature independently.
2. Predefined three algorithms (SHA1, SHA256, and GOST R 34.11-94) with identifiers for use in XML.
3. Added possibility to specify your own algorithms (see README.md and lib/signer/digester.rb)
4. Added possibility to change signature algorithm identifier
5. Detect GOST keypair and set signature digest algorithm and signature algorithm identifier appropriately
Various digests support: fix typos, rename some methods for convenience
Extract (re)setting algorithms defaults to separate method
Merge pull request #3 from Envek/more_digests

Allow to sign with another digest algorithms