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
try with:
```ruby
def algorithm(element)
  return unless element.is_a?(REXML::Element)
  algorithm = element.attribute('Algorithm').value
  algorithm.upcase!
  algorithm = 'SHA1' unless OpenSSL::Digest.constants.include?(algorithm)
  OpenSSL::Digest.const_get(algorithm)
end
```
I just added an eager return and renamed element.