Improvements for REPL-driven development

This commit does the following:

* Adds a gitignore for `dev/user.clj` so that developers can create
  a file there w/o worrying about it getting tracked in VCS
* Modifies `dev/user-repl.clj` so that developers can override the
  default REPL Puppet Server config settings in their own `user`
  namespace.
* Provides an example `user.clj.sample` file, which can be copied
  to `user.clj`, which has an example of how to override the
  config settings plus a convenience function for loading the
  `user-repl` namespace and launching Puppet Server.
* Modify `project.clj` to define `user` as the default REPL
  namespace.