Standardizing how we refer to installed directories

Standardizing on:
[OXYGEN_DIR]
[OXYGEN_PLUGIN_DIR]
[FRAMEWORK_DIR]
[DBXML_DIR]
[JDK_DIR]
[CUSTOMIZATION_DIR]
[HOME_DIR]
[OUTPUT_DIR]
[DITA_MAP_BASE_DIR]

Also fixing markup of editor variable names from <filepath> to <varname>
and examples form <filepath> to <codeph>.

Updating the readme with these rules.