Convert to using JARs as default

This commit converts the engine to use various JAR files and a cache of
resources held within them (see Arpk::Resources in lib/utils.rb).

In addition to expanding upon previous abilities to load resources from
JAR files, this commit also adds the ability to load resources by their
name from any JAR file present in the data/ directory so long as a
request to a supporting method begins with either "res://" or "@".

Please note that you may also explicitly reference the URI of a resource
via its full or partial path by naming its containing JAR file (ex:
"path/example.jar!MyResource").  This is the same format returned via
Resources.find().
Merge pull request #6 from florf/db-plugin

Confirmed.