[nemo-qml-plugin-email] Cache unread count for accts and folders

When there is large amount of messages DB queries can be expensive,
models that can be binded to UI elements need to provide fast response in order to
avoid UI freezes, this commit is a intermediate step, all DB calls should be moved
to a separated thread.
Merge pull request #99 from nemomobile/folderMOdel

[nemo-qml-plugin-email] Cache unread count for accts and folders