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.
They're rippled/ripple-lib integration tests. The fact that we use `npm test` to launch the script defined in `package.json` is not really important. I'd call them "mocha tests" before "npm tests," but still prefer "integration tests," which seems to provide more useful information.