Clean up attributes, metadata, README

* Set metadata to point to EverTrue
* Add `nginx` as a dependency to metadata
    * The `nginx_proxy` recipe needs it, so it’s
a dependency
* Clean up attributes
* Rewrite README:
    * Represent attributes more cleanly
    * Set a better example for using the cookbook
Switch license to Apache 2.0

OSS code is OSS
Clean up nginx_proxy recipe

Consistency is key

* Align resource attributes
* Use strings for modes
Move url_version attribute into recipe

Dynamically assigned attributes like these are best
set inside recipes.

In this way, we could change the version attributes
in a wrapper cookbook, and not need to also set
this attribute.
Clean up default recipe

* Align resource attributes
* Remove potentially inaccurate comment
    * Depending on attribute settings, it might not be
installed in `/opt`
Upgrade to Archiva 2.1.0