Various bug fixes.

Implemented various bug fixes:
- Removed unnecessary 'use' statements from various classes.
- Changed typehinting for all references of
  'Codesleeve\Stapler\UploadedFile' to 'FileInterface'.
- Added @throws annotations for methods that throw exceptions.
- Fixed various spelling mistakes.
Add ./idea folder to gitignore.

Added the ./idea folder that's generated by phpstorm to the .gitignore
file.