Support for HTTP-POST binding

[samuelcampos]

This pull request intends to add support for HTTP-POST binding for login and logout operations. However, it is not completed yet. It still misses:

• Support logout with HTTP-POST binding
• Unit tests

Before I spend more time implementing the missing parts, I want to check with you @pitbulk if it is OK to add support for HTTP-POST binding to this library.
I found another Pull request where this discussion has been made and I'm not sure what your plans are regarding the HTTP-POST binding.

Should I finish implementing this? Or this is a feature that you don't want to support at all?
If you want me to finish it, please take a quick lock on the current changes to validate my approach, so I can adjust anything.

Thank you,
Samuel Campos

[coveralls]

Coverage decreased (-2.6%) to 92.656% when pulling 6d95d4d on samuelcampos:master into 94b3ed8 on onelogin:master.

[pitbulk]

I don't have plans to officially support and maintain HTTP-POST binding for AuthNRequest (sent), LogoutRequest(sent and receive) and LogoutResponse(sent and receive).

I'm ok to merge reasonable changes on the core component, as the one you did on addSign, to make easier that feature, and keep them on the master branch, but changes on the toolkit component as you did will not be included on master branch.

I can offer to you the same solution that offered at the php-saml toolkit.

[samuelcampos]

Thank you for the transparency regarding your plans for this library.
I'll continue to implement support for HTTP-POST binding on my spare time and, at the end, I'll decide if it is reasonable to maintain the required changes on a separated branch.