Fix for inventory scripts with no path prefix that are in CWD.

Without this, using '-i ec2.py' will not work if '.' is not in PATH.
Merge pull request #2582 from blair/inventory-scripts-not-in-path

Fix for inventory scripts with no path prefix that are in CWD.
Catch recursive templating errors and display what string caused them.
Allow any parameter in a play to be set by --extra-vars through cleaner use of data structure
templating.
Complete streamlining of vars handling in playbooks and how those are templated to avoid
repeat template calls at playbook intialization time.
Much requested feature -- allows relative imports of content within roles or relative to any task or handler include (../templates for template ../files for copy)
Added examples of how roles work!
Demo of parameterized roles!
Remove stray file