Validate users before inviting
Swapping alphabetical pagination gem

We need the alphabetical pagination logic to take
care of the queries containing a join for the upcoming
organisation admin role. there is a need to scope users by
the first letter as well as the organisation they belong to.

The earlier gem doesn't allow us to specify something like
users.name, which we will need when we join with organisations,
or else mysql gets confused.
Adding Organisation Admin role

Organisation Admins can do everything that Admins can,
but only limited to their Organisation subtree, i.e.
their organisation and children of their organisation.
Moving config to right place
Authorisation around suspension, and misc changes
Using Cancan helpers to simplify controller
Organisation admin must belong to an organisation
Refactoring to create a User.filter scope
Cleaner; better loading for roles, abilities

Moving all roles related logic into separate classes
under Role and out of the User model. also, removing
the magic around Rails autoloading files.