Support a "no_server" flag in test config.

* Will use a running instance of rippled (possibly in a debugger).
* Modify all tests to respect the server_default value.
* Fail test if new account already exists and has a balance.
* README.md with instructions for advanced test debugging, particularly using no_server.

Reviewers @sublimator, @rec. Optional @wltsmrz.
Right, `m` is the result that is provided to my callback function if the `success` event is fired. eg. https://github.com/ripple/ripple-lib/blob/develop/src/js/ripple/transaction.js#L375

I reused the name out of laziness, but since it's a problem, I changed it to `result`. That seems more self-explanatory.