Unify Anime/MangaNetworkTask

They are doing the same, so they can be easily unified. This makes less
and cleaner code.
Also changed GET and GETDETAILS tasks to return Anime/Manga objects
instead of ArrayLists with only one element (now possible as the task's
return type is Object since #176).
Implement NetworkTask cancelling

Don't continue loading data if the user selects an other list. This
prevents unnecessary loading of useless data and fixes #190.

Exception are FORCESYNC-tasks, there is no reason to interrupt them.
Instead let them finish but don't display results (thats why there is
the new "cancelled" parameter in the callback function).

Cancellation code is a little more complex as it could be because of our
low minimum API level. It can be simplified as soon as we drop < API 11.
Only display network error info if it is actually a network error
Merge pull request #192 from d-sko/networktask_improvement

Networktask improvements