Friendlier error when using wrong --dev-mode params.

User is told in friendly english if she forgot to pass the server ip
when using development mode. Now development mode automatically
disables STUN check and UPnP.

Updated usage() string.

Now usage() is printed with new command 'openbazaar help'
unrelated pep8 fixes
fix on dev mode and private ip check

make one node a seed when on --dev-mode, let the other N-1
nodes know about the one seed.
Merge pull request #817 from gubatron/friendlier-dev-mode-error-and-help-command

Friendlier `--dev-mode` error message, new `help` command