Fix #102: Attachment's validate() is messed up

- removed printed exceptions
- made method abstract in Attachment
- made methods in subclasses protected

Signed-off-by: Victor Boctor <vboctor@microsoft.com>
Fix #29: Improper type check in ExchangeService.bindToFolder

- fixed type-check as inteded in #11
- fixed Exception message

Signed-off-by: Victor Boctor <vboctor@microsoft.com>