Fixing java.util.concurrent.RejectedExecutionException

Fixing a crash when a download task is rejected from the tread pool.
Merge pull request #76 from dglinenko/master

Fixing java.util.concurrent.RejectedExecutionException
Merge branch 'master' of https://github.com/koush/UrlImageViewHelper

# By Danil Glinenko
# Via Danil Glinenko (1) and Koushik Dutta (1)
* 'master' of https://github.com/koush/UrlImageViewHelper:
  Fixing java.util.concurrent.RejectedExecutionException