Adding supported for Encrypted SAML

[ksinkar]

These changes are for adding support for encrypted SAML messages. xmlenc from has been used for encryption and decryption. All existing tests for ruby-saml are passing. I had to upgrade the timecop gem though, for the tests to pass on my local machine. Unfortunately these are failing on Travis CI for REE and 1.8.x

@buffym @googya, could you please check and let me know if this works for your cases of Ping Federate and Encryption otherwise.
#9 #193 #186 #172

[pitbulk]

Thanks @ksinkar, we will review all this code.
Notice that we already created a branch with some decrypt functionality.

We will integrate in the ruby-saml gem the encrypt/decrypt functionality for sure.

[ksinkar]

@pitbulk, I had a look at the decrypt branch in which you are developing the decrypt functionality. It seems that branch is only able to handle checking of signatures for encrypted assertions. I don't see any code for decrypting the encrypted assertions.

Please correct me if I am wrong in the above inference.

I am eager to get the decryption functionality into this gem. People have been asking for this functionality for quite some time now. I suggest that you merge my changes into master or the decrypt branch as you see fit. As for the failing tests, it is dependency problem of nokogiri which I have tried to solve by updating the xmlenc gemspec as well, but not yet been able to solve it. If you have any ideas please let me know and I shall solve them pronto.

[pitbulk]

@ksinkar Here is the code to decrypt assertions:
https://github.com/onelogin/ruby-saml/blob/decrypt/lib/onelogin/ruby-saml/response.rb#L258

[pitbulk]

Replaced by #241. Right now we avoid the use of xmlenc.