Updated the documentatin of SchedulerDriver::stop().

Review: https://reviews.apache.org/r/23143
Temporarisly disabled MasterTest.OrphanTasks tests due to MESOS-1543.
Fixed spelling of "Authentication" in CHANGELOG

Fixed spelling of "Authentication" in CHANGELOG for 0.19 slave authentication feature.

Review: https://reviews.apache.org/r/22257
Improved the checking for libnl3 in configure.ac.

Review: https://reviews.apache.org/r/23145
MESOS-390 Fetcher supports file://// URIs
Addressing review comments