Add cancelJob() family methods

Add below methods to cancel not running jobs:


    boolean cancelJob(long id, boolean isPersistent)
    boolean cancelJobInBackground(long id, boolean isPersistent)
    boolean cancelJobInBackground(long id, boolean isPersistent, AsyncCanelCallback callback)


I know there is ongoing discussion at #2.
In other hand, I think it is also good to have ability to cancel job
with id, not only tag.

We encounted to necessity of removing job, only while the activity is
alive. In this case, cancel with id might be clearer than tag.

2052f5c is commit for migration to gradle project aware of latest
Android Studio IDE :).
This changes directory structure to gradle style one.
('examples' directory is not touched. sorry for TODO!)
If not ready to migration, I'll remove that commit.