WellManager get_current() can now use title or slug

The method now takes kwargs for `title` or `slug`, but defaults to title if using an arg to maintain backward compatibility. If no parameters are given, it raises a `DoesNotExist` error.

As note on armstrong/armstrong#74 regarding passing arbitrary kwargs into `.filter()`:
* I don't think we want or need the flexibility
* The actual title/slug exists on WellType not Well, so the filter parameter needs to be `type__title` or `type__slug` and that's not intuitive.