grid: don't refresh when no network connected

request ACCESS_NETWORK_STATE permission
refactoring: move refresh functions from FrupicFactory to RefreshService
refactoring: job/jobmanager service for refreshing
JobManager: use JobWorker threads to process new jobs
factory: fix concurrent download of the same frupic by using different tmpFiles

if a frupic.tmp.0 is detected, go on with frupic.tmp.1, etc.
should fix race condition when downloading a frupic twice, possibly resulting
in corrupted frupic image files
preferences: try to fix crash on activity close while cache tasks are running
factory: use up to 5 concurrent http clients

was: 4
jobManager: skip jobs that are cancelled before run
jobManager: add priority to jobManager.post
grid: fetch previews using JobManager instead of PreviewFetchTask

use up to 5 worker threads, animate only currently loading Frupics in grid
store DefaultHttpClient in worker thread, supply to Job
RefreshJob: use DefaultHttpClient from worker thread
grid: more aggressive loading animation
job: add OnJobProgress callback
factory: increase buffer size for fetching to 16kb