Add constants

```
Add :

WP_CONTENT_DIR  // no trailing slash, full paths only
WP_CONTENT_URL  // full url 
WP_PLUGIN_DIR  // full path, no trailing slash
WP_PLUGIN_URL  // full url, no trailing slash
```

Original issue reported on code.google.com by `i...@mindsharestudios.com` on 5 Jun 2012 at 7:57
