Build.get_downstream methods

These methods do not return the proper data.  Fixing them and adding a
unit test for them but i'm not familar with magicmock so its having an
issue.
Keep the same scheme (protocol) as was used in Jenkins constructor in all requests

Some Jenkins instances are configured to allow reading the data over HTTP scheme, while for manipulating with
data (e.g. create or delete job) HTTPS scheme is required. Allow user to specify preferred URL scheme (by
specifying Jenkins URL) and use this scheme also manipulations with jobs.

This was introduced in 4f29f6742410736016b568002d8569f19d786841 (version 0.1.13) but lost again in
79934f0b5ca44a1d4ecbaf534eb1add8e343193f.
Add kerberos authentication requester using requests_kerberos
Merge branch 'master' of https://github.com/rocktavious/jenkinsapi into unstable
Merge branch 'unstable' of https://github.com/salimfadhley/jenkinsapi into unstable