Leftover / Orphaned environment variables in child shells

the problem is simple:

``` bash
$ echo $MAYA_LOCATION

$ rez-env maya
> $ echo $MAYA_LOCATION
/usr/autodesk/maya2014.51-x64
> $ rez-env nuke
>> $ echo $MAYA_LOCATION
/usr/autodesk/maya2014.51-x64
```

rez makes no effort whatsoever to remove old variables.

This is a pretty big problem. what if you drop into a shell with a resolve that sets `LD_LIBRARY_PATH`, then drop into another shell below that which does not. `LD_LIBRARY_PATH` is still set, potentially along with dozens of other variables which are not relevant and potentially harmful to the current resolve.

I believe if we can keep track of which variables rez sets, then we can unset all of those before the next resolve. This is one of the reasons that I'm not a fan of the `source` command, since it can wreak total havok on the environment in ways that rez cannot easily track (aside from the fact that it breaks promise of parity between various shells).

Start at the rez-env entry point and inspect how child-shell environments are assembled, especially the handling of variables set by a previous resolve. Done means variables from an outer resolve do not remain when a nested resolve does not define them, while behavior remains consistent across supported shells.

Written by the indexing model from the issue text.