Allow adding of appenders as objects

Previously, appenders could only be added by specifying the filepath
to the appender. This required the appender's path to be specified
either relative to the log4js installation, relative to a NODE_PATH
token, or absolute. This creates a coupling between the log4js
configurer and the log4js installation location, or a coupling between
the log4js configurer and the global NODE_PATH. This commit removes
the coupling by allowing the loading of appenders to be done relative
to the log4js configurer.
Simplied loadAppender logic.
Merge pull request #189 from jengler/add-support-for-prerequired-appender-styles

Add support for prerequired appender styles