PE-246: remove advice about security implications of environment variables

This removes advice, especially security advice, about using environment
variables in favour of strictly advising that it is possible.  Given potential
leaks of that information, this seems like the best course of action.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Merge pull request #111 from daniel-pittman/bug/master/pe-246-suggesting-environment-variables-is-risky

PE-246: remove advice about security implications of environment variabl...