create_pramas method for HTTP-Post Binding

Allows ease of using a HTTP-Post binding, the client call call create_params get a hash of parameters to use in the html form.
Now the caller does not have to decode and parse a URL to get the SAMLReqeust for use in the form.
XMLSecurity::RequestDocument class

Extends REXML::Document to add XML Signature functionality.
Preform Authrequest signing if settings specify request signing

New settings were added to hold XML Signature attributes
XML signature support for LogoutRequest
Request signature test cases
Update docs to demonstrate signature functionality
fixes test failure

tests were looking for extra prams at the END of the request URL.
SAMLRequest needs to be URL-encoded
Allow metadata generation to publish the cert if request signing is specified.
Setting support for assertion_consumer_service & assertion_consumer_logout_service
handle SimpleSign request

Conflicts:
	lib/onelogin/ruby-saml/authrequest.rb
	lib/onelogin/ruby-saml/settings.rb
	test/request_test.rb
add some test
Resolve conflicts in pr/127
MERGE one-login/ruby-saml master at 34a43c9823b9c41ec348edd1394573260d27df52

and resolve conflicts.
Add support for IdP initiated logout (SLO)

Prior implementations of ruby-saml support SP initiated logout. This
adds support for IdP initiated logout.

The flow is, the IdP sends a Logout Request to the SP, and the SP
returns a Logout Response. These are implemented in the
Slo_Logoutrequest/SloLogoutresponse classes respectively.

I also added a SamlMessage class which contains some common code for
all the Saml Messages, both requests and responses, for example XML
validation and encoding/decoding.

I couldn't decide if I should have separate classes for IdP vs SP
logout, but kept them in their own classes for now. I'm happy to look
at combining them with the existing logoutrequest/logoutresponse
classes if that is deemed more appropriate.
Update test data to generic fields
LF
Add an additional assertion to valid test
Fix test for 1.8.7
Merge PRs #159 and #162