added env command to add environment variables

environment variables added with `duckpan add <name> <value>` are stored
in $app->cfg->cache_path/env.ini
read in env.ini variables on server startup
list env variables & values with `duckpan env key`
moved env variable initialization to DuckPAN.pm
print error and exit if variable is already defined
move to $config_path/env.ini, and error handling

create the file if it doesn't exist, and don't choke when it's empty
add duckpan env to Help
add duckpan env rm <name> command
error w/o key to rm, & don't output on success
added `duckpan env rm <name>` command to help
Merge branch 'env'

Conflicts:
	lib/App/DuckPAN/Help.pm