[nemo-qml-plugin-email] Add cached unread count of current active folder.

This commit exposes to properties to QML currentFolderId and currentFolderUnreadCount.
Update currentFolderId when you change a folder then currentFolderUnreadCount
will tell you how many emails that folder has unread status.
Merge pull request #55 from rainemak/exposeUnreadCount

[nemo-qml-plugin-email] Add cached unread count of current active folder...