[nemo-qml-plugin-email] Add CONFIG+=OFFLINE. Ignore all actions that need network access.

When CONFIG+=OFFLINE is defined any action that needs network access will be silently
ignored, all actions that can be performed offline will work in the same manner as without
this option.
Merge pull request #78 from nemomobile/offline-support

[nemo-qml-plugin-email] Add support for offline builds