Adding download progress block on demand

There is some overhead incurred when tracking the progress for a http task. This doesn't add the download progress block until one of the progress methods are called.
Cleanups
Removing unneeded import
Merge branch 'develop' into feature/add-download-progress-on-demand

Conflicts:
	Zinc/ZincDownloadTask.m
Merge pull request #31 from mindsnacks/feature/add-download-progress-on-demand

Adding download progress block on demand