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.
I didn't add the two new functions to `exports`, because I didn't figure they'd be useful outside of the file. Also, because I forgot about the separate `exports` section! 

Defining the function this way means you have to fully specify `exports.create_account` to use it locally.

I solved both problems with
```
exports.create_account =
create_account =
function(remote, src, account, amount, callback) {
```