Created Snapshot and Prerelease classes

Created Snapshot and Prerelease classes

Snapshot and Prerelease classes provide simple OO interface to Snapshot
and Prerelease metadata. Previously get_repo_data function was used, but
it was hard to read and snapshot and prerelease data was mixed there.
That complexity was the first reason to replace it with Snapshot and
Prerelease classes.

Another reason was snapshot increment. It would be quite hard to
implement it using functions. Object oriented approach makes it simpler
to implement and more readable.

Change-Id: I1f77e3de047cd6666df30a3f40040115a9554b1f
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>