All public methods shall start with tcfg_

Using a special prefix well help to ensure this module
has no collisions with existing methods when it is mixed in.
This is important for methods like 'set' and 'get' which are
very likely to collide with something in a base class.

The idea going forward will be to use TCFG_ prefix by convention
for
  * env var overrides
  * method names
  * config file names
Added two more tiers:

- Config file
- Environment variables

Realized this implementation is too generic.  Need to consider
that each tier is handled in a special way, so should rip out some
of the meta programming I think