Covers from inner folders

Bugfix
---
* Folder-specific Configuration is now applied correctly

e.g: if you called Teca this way
```
alfateam@broken ~/projects/Teca
$ python teca.py ../../screens/
```
you received a lot of warnings, because Teca appled "../../screens" before the real name of the folder, so configuration for a specified folder was not recognized (and therefore applied).
Now it's fixed.

* No more broken Covers images

In case no images were contained in the folder, a void (=> broken) "cover_.png" file were created.
From now on, the image is randomly chosen between the images in the inner folders. In case no images are present, a default image is used as cover image.
The path of the default image can be set in the configuration file (_default_image_ keyword)

Misc
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* Restored "templates/default.html" as default theme