Open web links in browser

[digitalnyc1]

Add the ability to open http:// and https:// links in the system's default web browser.

Also adds the "weblinksafety" config variable:

• When True, use the play.net link redirection warning page. (default)
• When False, open the link directly in the browser.

Resolves #99

[mj-colonel-panic]

Looks good!