dss_sig_value: Pure python conversion of (r, s) to byte stream
use pyasn1 for creating the dss-sig-value structure
make DSSSigValue class private
rename dss_sig_value -> der_encode_dsa_signature
we should use der rather than ber

Doesn't make a difference in this specific case but it is correct.
negative test
Merge pull request #980 from reaperhulk/dsa-sig-serializer

dss_sig_value: Pure python conversion of (r, s) to byte stream
Fixed C style guide violation
Fix two more violations
Foolish hobgoblin is best hobgoblin.
Add DSA verification
Add verification tests
Add DSA verification docs
fix pep8 violation
restructure some tests, add skip, use der_encode_dsa_signature
remove dsa_signature_from_components since it's no longer needed
remove hash check in verifier as this isn't a proper restriction
add interface docs for dsa_params_supported
Merge pull request #981 from alex/c-style-guide

Fixed C style guide violation
dsa_hash_supported needs to chech hash_supported