CCLE-4304 - Open source Site invitation

* Current version of the UCLA version of the invitation enrolment plugin. Still need to remove UCLA specific code.
CCLE-4304 - Open source Site invitation

* Stripped out UCLA specific code.
CCLE-4304 - Open source Site invitation

* Fixed upgrade from previous version of enrol_invitation.
* Removed references to "site invitation" to just "invitation".
CCLE-4304 - Open source Site invitation

* Stripped out references to UCLA specific methods.
* Changed config setting that handled invitation expiration so that it does not conflict with previous version of the plugin.
CCLE-4304 - Open source Site invitation

* Fixing Moodle 2.5 specific form logic.
* Reworked version numbers so that they are aligned with the UCLA specific branch.
* Fixed form warnings.
CCLE-4319 - Create README

* Wrote README with installation and setup guide.