Change log level of a debug message from warn to debug
Add an optional description to the manifest
Merge pull request #12 from gitblit/fix_loglevel

Change log level of a debug message from warn to debug
Merge pull request #13 from gitblit/description

Add an optional description to the manifest
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().