Pesos

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* Implemented a ZookeeperMasterDetector. Just pass a 'zk://ensemble/path'
URI to the PesosSchedulerDriver and it will automatically be selected. Must
use 'pesos[zk]' requirement for this support. By default 'pesos' can only
talk to explicit PIDs.

* Implements reliable scheduler registration.

* Add basic test coverage for the scheduler.

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* Pins to ``compactor==0.2.2`` which fixes a number of race conditions.

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* Add stout's ``Duration`` support to pesos so that it works correctly with
slave checkpointing.

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* Improved executor test coverage.

* Vagrant image added for integration testing

* Revendored mesos protobufs for 0.23.x

* Implement against mesos.interface instead of vendorized interface in ``pesos.api``
(``pesos.api`` has now been removed.)

* Example executor and scheduler implementations provided.

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* Basic framework implemented with vendorized API and mesos interfaces.