Comfortable Cosmetics

In continuation to travis-ci/travis-cookbooks#231 discussion. Beside `tarball.rb -> default.rb`, all these changes should be backward compatible.

**Not backward compatible propositions:**

*to be discussed... (not necessarily part of this PR)*

* Unify `neo4j` (used for system username and init.d script name) vs `neo4j-server` (used for cookbook name and directory/file names). I'd propose to rely on `node[:neo4j][:server][:name]` attribute everywhere.
* About `tarball.rb -> default.rb`: I went a bit fast here, let me know if you prefer that I drop this commit and rebase.
* (Re)introduce checksum validations for remote tarballs?