Inline images distinguished from attachments

This should be all of the changes. I removed that test email and the unit test which used it. The other unit tests have a file path specified which is specific to my machine so you would need to update that path for your own machine otherwise they will fail.

There are a couple of unit tests failing now but I think this is from the main project and not as a result of these changes.

The changes here are not currently suitable for submitting to the main project yet due to:
1. the new unit tests have those paths specified in them. Those unit tests can probably be written in a better way. Also need to check whether the added unit tests cover all of the changes made.
2. The new code does not have the xml documentation comments which are present in the rest of the project.
3. I don't think that @foens wants a "body" field to be included in the Message class. This is required for the Inline image checking to see whether an image attachment is referenced in an img tag. Instead of using this "body" field, we could just search the first html part or all of the html parts which are not attachments.