Add Downloader implementation with tests

Add Downloader implementation with tests

DownloaderImpl is responsible for parsing Job's config and searching
for information about files (artifacts) related to the test.
It delegates to ArtifactsManager downloading images and testfiles
required to run Job (Push). It creates also paths for output
artifacts (Pull). It updates Job's config with proper paths
for these artifacts.

It does also monitor process of downloading files in ArtifactManager
and finally it notifies Controller when all files are downloaded
or in case of failure.

Change-Id: I578108dd1a924aaf36435e4dbc5d8dcce50214ee
Signed-off-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>