Plugins, DBSession

Adds prototypal support for the loading of "plugins;" ruby source files
which may be used to extend the Arpk namespace and engine.  This process
is performed automatically once the environment variables are loaded,
using the "plugins" entry.  You may manually specify which plugins to
load ahead of time with the --plugins bootstrap switch, or by calling
Arpk.plugins( ... ) directly with a list of filenames.

DBSession is an encapsulation of DB as a class, allowing for multiple
simultaneous connections to separate databases.  This is useful for such
scenarios as maintaining a connection to the asset store while also
writing a save file.