Ensure marmottawrapper has minimal external dependencies

* Use `#{app_root}/marmotta` as the data directory...for real
* `marmottawrapper` now no longer depends on `curl` and `unzip`
    * `rake marmotta:fetch` uses `open-uri` and grabs a tarball
    * `rake marmotta:install` uses `Archive::Tar::Minitar` and `GZip`
* Update README to describe Marmotta configuration conventions 

Specifically, once Marmotta creates its required files on its first startup, users can then modify the default configuration.