New TC 8.x ArtifactWrapper & ArtifactDownloader

Created the new ArtifactWrapper2 and  ArtifactDownloader classes that
support the new TeamCity 8.x REST API.
Updated README.md with new BuildArtifact method
Merge remote-tracking branch 'upstream/master'
Merge pull request #1 from vmandy/master

pull from vmandy:master