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
We should not be parsing and manually formatting certificates. Instead, we should use OpenSSL. Something like:
```ruby
raw = File.read "cert.cer" # DER- or PEM-encoded
certificate = OpenSSL::X509::Certificate.new raw
```

```format_private_key``` and ```format_cert```  functions are later used with OpenSSL, e.g.:
```ruby
formated_private_key = OneLogin::RubySaml::Utils.format_private_key(self.private_key)
private_key = OpenSSL::PKey::RSA.new(formated_private_key)
```

If ```self.private_key``` is already PEM formatted, it seems you can avoid this completely by doing:
```ruby
OpenSSL::PKey::RSA.new File.read 'rsa.pem'
```