Fix assumption that plugins folder is in the wp-content folder

If you have defined WP_PLUGIN_* outside of WP_CONTENT_* the path to the current plugin that is including CMB_META_BOX is incorrect.

Didn't touch windows path, but similar change may be required.
Merge pull request #339 from davidosomething/patch-1

Fix assumption that plugins folder is in the wp-content folder
Merge 0.9.4 from trunk and update readme with props
Fix broken props urls
bump trunk to 0.9.5