Merge 159 and 162

In this PR I will merge the PR #159 and #162 and try to add signature to logout request and response
Drive-by comment: The SignatureMethod should include the public key algorithm used and not just the digest method. For example:

   http://www.w3.org/2000/09/xmldsig#rsa-sha1

See section 6.1 of:
   http://www.w3.org/TR/2009/WD-xmldsig-core1-20090226/

has the latest possible method.

FYI, I recently forked a new gem into https://github.com/awong-dev/nokogiri-xmlsec-me-harder to handle xmldsig/xmlenc via libxmlsec1 to avoid needing to reimplement the crypto application. I'm wondering if it'd be worth swapping out The XMLSecurity module for this gem. Fewer crypto implementations in the world is usually a better thing.