Grant role with unit tests (#762)

• Adding grant_role and updating README

• Add unit tests for grant_role

• Define relationships with role and group

But only if they are defined, similar to other defined types in this
module.

• Add connect_settings to grant_role

Only require Postgresql::Server if it is not specified. This check might
change, depending on MODULES-3229.

• Document grant_role/connect_settings

• Add 'ensure' to grant_role

Does not affect the relationships with Role[$user] and $Role[$group].
They don't have have an ensure argument (yet), so there is no situation
where you would first revoke the membership and then drop the role.

• Fix variable typo (user => role)

• Use correct quotes for identifier

• Add validation for group and role

• Puppet 3 treats numbers like strings