Implement rollback method.
Implement rollback in BaseImporter class.
Bump version.
Set REJECTEDFILE to __debug__. Add REJECTMAX option.
Add _get_num_rejected_rows method. Make get_errors method return the number of errors instead of bool.
Update readme.
Fix docstring.
Pull-request feedback changes.
Only create _rejected_file_obj when used.
Merge pull request #13 from brocaar/add_rollback

Add rollback and error count