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
Add filter for fallback field values
Update readme with props
Merge 0.9.4 from trunk and update readme with props
Fix broken props urls
Added functions needed for the timezone related fields.
Fixed merge conflict
Added date time picker with time zone combo field
Added select timezone dropdown field
Merge