Allow SII compliance without forking

[Gallaecio]

Fixes #64. #52 could probably be considered fixed by this as well.

List of changes:

• XMLSigner gains a new method, key_value_serialization_is_required(), which
  may be overriden in subclasses to force key value serialization regardless of
  the existence of a certificate chain.
• The C14N algorithm transform is generated regardless of the signing method.
• XMLProcessor becomes a new-style class.

These changes make it possible to define an SII-compliant XMLSigner subclass as
follows:

from signxml import methods, XMLSigner

class Signer(XMLSigner):

    def __init__(self):
        super(Signer, self).__init__(
            method=methods.detached,
            signature_algorithm='rsa-sha1',
            digest_algorithm='sha1',
            c14n_algorithm='http://www.w3.org/TR/2001/REC-xml-c14n-20010315')

    def key_value_serialization_is_required(self, cert_chain):
        return True

[codecov-io]

Current coverage is 94.94% (diff: 100%)

Merging #65 into master will decrease coverage by 0.46%

@@             master        #65   diff @@
==========================================
  Files             3          3          
  Lines           588        514    -74   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits            561        488    -73   
+ Misses           27         26     -1   
  Partials          0          0          

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[kislyuk]

Thanks. Please add a test where key_value_serialization_is_required is overridden in a subclass.

[kislyuk]

Closing due to lack of response.