Separate `basePath` from `baseUrl`

- `basePath` is now a fully separate concept from `baseUrl`
- `basePath` is a general Intern configuration parameter (vs. `baseUrl`, which is a loader parameter) that declares the root path from which files will be served
- `basePath` is only directly relevant to Node (runner, createProxy); it's ignored by the browser client. If not specified it defaults to process.cwd()
- If a relative `basePath` is specified in the intern config, it is relative to process.cwd()

fixes #249