**Bugfixes and improvements:**
- SimpleEmail.body_from_file() and HTMLEmail.body_from_file() methods are now static. It removes inconsistency in assigning. Now all attributes (body, attachments, subject, recipients) are now assigned via `op.<property> = <value>` type of expression.
- Fixed an issue with BodyError message not being formatted as expected.
- Filepaths (attachments, etc) have been converted into POSIX-like paths (with forward slashes as separators) instead of being system-dependent. It insures greater readability and prevents some unexpected behaviours.