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
```ruby
def algorithm(element)
element = element.attribute('Algorithm').value if element.is_a?(REXML::Element)
element.upcase!
element = 'SHA1' unless OpenSSL::Digest.constants.include?(element)
OpenSSL::Digest.const_get(element)
end
```
unless `algorithm` is also receiving `Fixnum`s, this will work