Preserved newlines for copy/paste in Firefox

Due to a difference in implementation, Firefox does not preserve
newlines on copy when output is styled with "white-space: pre-wrap;"[1].
Firefox does preserve newlines if the output is wrapped in <pre><code>.
Switching to the semantic markup fixes Firefox and functions the same
as before in Chrome and in Safari.

This change addresses MESOS-661.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=116083

Review: http://reviews.apache.org/r/13946
Fixed a bug in SlaveRecoveryTest/*.MultipleFrameworks test.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/13837
Fixed the State output operators.

Review: https://reviews.apache.org/r/13954
Fixed a CHECK failure in the master when successive exited events
occur for a disconnected slave.

Review: https://reviews.apache.org/r/13956
Fixed a bug in reconciliation that failed to account for unknown
executors.

Review: https://reviews.apache.org/r/13921
Added a cgroups::memory namespace.

Review: https://reviews.apache.org/r/13903
Fixed CgroupsIsolator to listen for OOMs of recovered executors.

Review: https://reviews.apache.org/r/13904
Exposing cpu.stat statistics in the monitoring endpoint.

From: Christina Delimitrou <christina.delimitrou@gmail.com>
Review: https://reviews.apache.org/r/13868
Ignored the recovery error due to the inability to
find the latest executor run.

Review: https://reviews.apache.org/r/14001
Update Powered By page to reflect new organizations using Mesos.
Adds new documentation page to include links to videos and slides for Mesos presentations.
Fixed the broken OsTest.process and OsTest.killtree tests.

Review: https://reviews.apache.org/r/14072