Add an optional requires manifest value

As integrators of pf4j evovle their extension APIs it will become
a requirement to specify a minimum system version for loading plugins.
Loading & starting a newer plugin on an older system could result in
runtime failures due to method signature changes or other class
differences.

This change adds a manifest attribute to specify a 'requires' version
which is a minmum system version.  It also introduces a method to
specify the system version of the plugin manager and logic to disable
plugins on load if the system version is too old.  This works for both
loadPlugins() and loadPlugin().

Add an optional description to the manifest

Change log level of a debug message from warn to debug

Allow disabled plugins to be unloaded and deleted

Allow inspection of disabled plugins by not stopping resource loading

The differences between a DISABLED plugin and a STARTED plugin are:

1. a STARTED plugin has executed Plugin.start(), a DISABLED plugin has not
2. a STARTED plugin may contribute extension instances, a DISABLED plugin may not

DISABLED plugins still have valid classloaders and their classes can be manually
loaded and explored, but the resource loading - which is important for inspection
has been handicapped by the DISABLED check.

Instead of preventing loading the extension indexes for DISABLED plugins, the
extension finder should only return ExtensionWrappers for STARTED plugins.

Enable disable plugin

Implement loading a single plugin archive

Implement delete plugin