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
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