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.

Tentative reviewers @sublimator, @vinniefalco.
It doesn't seem like you're adding `create_account` to `exports`!

The whole "define the function, then add it to exports" thing seems needlessly wordy - why not just:

`exports.create_account = function(remote, src, account, amount, callback) {`