Fix slow startup

Closes #253:

 - The file traversal ignores directories that can't be accessed. Node Inspector still prints a message to stderr, so that it's easier to detect this problem.
 - If the project root does not contain package.json, only well-known subdirectories are listed:
  `./*.js, ./lib/**/*.js, ./node_modules/**/*.js, ./test/**/*.js`

Closes #237

 - Add caching of the last file traversal result to prevent redundant traversals. Note we can't use glob's cache/statCache because it uses relative paths.
 - Optimise parsing of source map URLs from the initial set of scripts - get rid of extra V8 requests.

@3y3 could you please review?