Update credentials plugin to 1.9
Test fetch and clone with credentials

Places repository URL, private key, and user name information outside
the repository so that the test private keys and repository URLs do not
need to be shared in the public repository.  That allows testing with
real repositories (like github and bitbucket) and real private keys
(like mine).

Uses a parameterized test which generates test parameters
by reading from ~/.ssh/auth-data/repos.csv.  In addition
to the repos.csv file contents, the test always uses the
https://github.com/jenkinsci/git-client-plugin.git repository for a test.

JGit version of this test does not exist yet.
Assert more conditions in authentication tests, prep for JGit test

There is still an open problem in the fetch_() test case for JGit.
It does not checkout the files, so the assertion for the existence
of the expected README.md file fails.  

That assertion is temporarily disabled.  It will be enabled once
the issue is understood and resolved.
Add JGit credentials test
Create a separate CONTRIBUTING file per recent suggestions
Use the correct link to the CONTRIBUTING file
Add getRemoteTagNames API entry point

From Niklaus Giger
Add test of a deprecated clone method