[buteo-syncfw] Fix memory leaks

OOP plugins must emit either success or error signal when they are done.
Not doing so results in memory leaks.

It appears that OOP plugin objects themselves have to be explicitly
deallocated. Not doing so results in memory leaks too.

Removed OOP-specific methods from the generic SyncPluginBase interface.
Merge pull request #68 from monich/oop-plugin-signals

[buteo-syncfw] Fix memory leaks