[nemo-qml-plugin-email] Add count and property invokables to attachmentlistmodel.
[nemo-qml-plugin-email] Add messageDownload() invokable.
[nemo-qml-plugin-email] Add methods to cancel single actions.
Merge pull request #77 from nemomobile/attachments

[nemo-qml-plugin-email] Add count and property invokables to attachmentlistmodel