Remove excluded dirs from scan extra dir list

[makao]

In large projects like for example Magento where there is a lot of cache and many media files this plugin doesn't work. All the time I receive:
Timeout while resolving completions!

This PR fixes codeintel_scan_exclude_dir setting and make this plugin work in large projects. See #49.

I'm not Python expert so it only checks first level folders. For example if your project structure looks like this

ProjectFolder
- app
- src
- var
- web

And your exclude setting looks like this
"codeintel_scan_exclude_dir": ["node_modules", "var", "web", "media"]
It will exclude var and web folder.

It also supports multiple project folders (Project -> Add Folder to Project).