Use field storage object for `exists` and `delete`

Using os is naive in regard to users of django-storages. Storage is not obliged to implement absolute path.
Thus, using file name instead.
Add gitignore file
Cleanup and additional checks
Ensure no errors are swallowed
Merge pull request #2 from beresovskiy/pullrq-storage

Use field storage object for `exists` and `delete`