Changed executor state recovery to allow run recovery in absence of executor info.

This patch let executor recovery recover runs in the absence of
executor info.  This is needed as new task-info patch will introduce
an intermediate state where the executor info hasn't been check
pointed. In this interim, the slave may fail-over and should be in a
position to clean up orphan containers (as for now, the containerizer
API doesn't provide a way to reconcile the executor info and it is
therefore not possible to recover the containers in this case).

Review: https://reviews.apache.org/r/20221
Fixed missing call to delete in Cluster.
Added missing #ifdefs for http parser in decoder.hpp.

Review: https://reviews.apache.org/r/20441
Changed registrar tests to use LogStorage.

Review: https://reviews.apache.org/r/20432
Improved Registrar performance by using a hashset on SlaveIDs.

Review: https://reviews.apache.org/r/19762
Fixed formatting in getting-started document.
Added containerizer.proto.

Adds containerizer.proto containing protobuf message definitions for
"Launch", "Update" and "Termination". All of those are used in the
upcoming External Containerizer.  These protos are defined within
their own C++ namespace: "containerizer".

Replaced slave::Containerizer::Termination with the
containerizer::Termination protobuf, gaining a serializable
Termination message.

The protoc results are added towards the Python Egg
(containerizer_pb2.py) and the Java package
(ContainerizerProtos.java).

Review: https://reviews.apache.org/r/20141
Added history to metrics library.

Review: https://reviews.apache.org/r/20015
Fixed public includes.
Fixed bug with generated headers.
Added braces to show locked section in Metric.
Fixed containerizer-header & proto installation.

Review: https://reviews.apache.org/r/20508
Fixed an incorrect CHECK in Group.

Review: https://reviews.apache.org/r/20470
Adds Viadeo to Powered by Mesos list.
Adds 6 recent Mesos presentations to documentation list.
Discarded the future returned by select when future.discard() is called.

Review: https://reviews.apache.org/r/20533
Minor punctuation/grammar cleanups.
Added Statistics for TimeSeries data.

Review: https://reviews.apache.org/r/20047
Fixed flaky test ReservationAllocatorTest.ResourcesReturned.

Review: https://reviews.apache.org/r/20580
Added Statistics support for Metrics.

Review: https://reviews.apache.org/r/20018