Add generic state API support for xAPI library

This issue covers the tasks to implement a plugin independant implementation for xAPI state REST functions. Those include:

• Implement a generic xapi_state_post and xapi_state_get events
• Add a validate_state method in the xapi handler base class that every plugin that sues xAPI states could override
• Add state request validation in the rest server
• Add a standard Moodle web services to get/post xAPI states from a specific plugin (needed for Moodle mobile APP)
• PHP Unit for all the APIs listed