Fix ECDSA signature encoding/decoding (#150)
Fix ECDSA signature encoding/decoding (#150)

According to https://www.w3.org/TR/xmldsig-core1/#sec-ECDSA,
"The signature value consists of the base64 encoding of the
concatenation of two octet-streams that respectively result
from the octet-encoding of the values r and s in that order."

Previously, the signature value was assumed to be a DER-encoded
signature, which is incorrect. There was already code for dealing
with a similiar situation for DSA signatures, but not ECDSA.
XML-Security/signxml,kislyuk/signxml,XML-Security/signxml