Feature: Configurable role search path

Feature discussed here: https://groups.google.com/forum/#!topic/ansible-devel/ITZJUnyzB1g

This adds an option to ansible.cfg: "roles_path" and the environment variable ANSIBLE_ROLES_PATH for extra locations to search for ansible roles to be made available to playbooks.

Existing behavior remains unchanged, new behavior is only around when the configuration option is set.

Tests all pass, let me know if I should change it up in any way.

Thanks,
Thomas Omans