Changed FragmentPagerAdapter to FragmentStatePagerAdapter

- Made ImageLoadJobs cancelable

The FragmentStatePagerAdapter allows old Fragments to be removed from
the memory. This allows nearly unlimited browsing through images without
memory issues.
Reduced the number of background jobs

This might be a temporary solution, but it avoids out of memory bugs
when scrolling very fast trough pictures. (too many ImageLoadJobs
running at the same time)
Merge pull request #37 from L-Henke/memory

Reduced out-of-memory errors when browsing images