JWT - Verify and update client API

[gallayl]

Update the JWT Service based on the changes in the related issue: SenseNet/sensenet#145

• JwtService.Logout() should fire a logout request to the backend
• Remove the X-Access-Data from requests - breaking change
• Re-validate JwtService based on the finialized implementation

We shoud wait until an API changes will be finialized, documented and a new version of sensenet Services will be released.