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
Simplified os::hostname() in libstout.

Review: https://reviews.apache.org/r/11108
Shuffle offers before sending to frameworks.

This helps ensure fair allocation of resources.

Review: https://reviews.apache.org/r/11121
Added --enable-debug configure flag.

Review: https://reviews.apache.org/r/11605
Slave ping timeout may be increased by flags.

Review: https://reviews.apache.org/r/12885
Turned down noise from ZooKeeper.

Review: https://reviews.apache.org/r/11127
Fixed broken build on GCC 4.8 due to -Wall.

With GCC 4.8, Boost fails to build from '-Wunused-local-typedefs', which
is implied by '-Wall'.

Review: https://reviews.apache.org/r/11974/
Use hostname instead of IP for redirection.

In the Web UI, the redirection does not work for hosts where the local
interface address is not publicly accessible.  For example, with EC2 the
redirection will not work.

This change writes the Master's hostname into ZooKeeper, thereby
allowing the redirection to work with private IP addresses.

Review: https://reviews.apache.org/r/11975/
On recovery fail, do rm -rf /tmp/mesos/meta.
Use libstout cache for slave completedFrameworks.

The list of retired (completed) frameworks is now a stout cache rather
than a boost circular buffer.  There are now key iterator accessors for
the cache class, and a removal member.

Review: https://reviews.apache.org/r/13599
Added a slave load hint for scheduler sorting.

With this change, the offers will include a slave load hint based on the
5 minute load average for that slave (returned from getloadavg()).
Frameworks can use this hint to select among otherwise similar offers,
as an optimization.

Review: https://reviews.apache.org/r/13675