Taskgraph

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* Fixing an issue where a Task would hang on a ``join`` if the number of
workers in TaskGraph was -1 and a call to ``add_task`` has a non-``None``
passed to ``target_path_list`` and the resulting task was ``\.join``\ed
after a second run of the same program.

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* Fixing an issue where TaskGraph would hang on a ``join`` if the number of
workers was -1 and a call to ``add_task`` has ``None`` passed to
``target_path_list``.

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* Taskgraph now supports python versions 2 and 3 (tested with python 2.7,
3.6).
* Fixed an issue with ``taskgraph.TaskGraph`` that prevented a multiprocessed
graph from executing on POSIX systems when ``psutil`` was installed.
* Adding matrix-based test automation (python 2.7, python 3.6, with/without
``psutil``) via ``tox``.
* Updating repository path to ``https://bitbucket.org/natcap/taskgraph``.

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* Auto-versioning now happens via ``setuptools_scm``, replacing previous calls
to ``natcap.versioner``.
* Added an option to ``TaskGraph`` constructor to allow negative values in the
``n_workers`` argument to indicate that the entire object should run in the
main thread. A value of 0 will indicate that no multiprocessing will be used
but concurrency will be allowed for non-blocking ``add_task``.
* Added an abstract class ``task.EncapsulatedTaskOp`` that can be used to
instance a class that needs scope in order to be used as an operation passed
to a process. The advantage of using ``EncapsulatedTaskOp`` is that the
``__name__`` hash used by ``TaskGraph`` to determine if a task is unique is
calculated in the superclass and the subclass need only worry about
implementation of ``__call__``.
* Added a ``priority`` optional scalar argument to ``TaskGraph.add_task`` to
indicates the priority preference of the task to be executed. A higher
priority task whose dependencies are satisfied will executed before one with
a lower priority.

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* Refactor of core scheduler. Old scheduler used asynchronicity to attempt to
test if a Task was complete, occasionally testing all Tasks in potential
work queue per task completion. Scheduler now uses bookkeeping to keep track
of all dependencies and submits tasks for work only when all dependencies
are satisfied.
* TaskGraph and Task ``.join`` methods now have a timeout parameter.
Additionally ``join`` now also returns False if ``join`` terminates because
of a timeout.
* More robust error reporting and shutdown of TaskGraph if any tasks fail
during execution using pure threading or multiprocessing.

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* Fixed a critical error from the last hotfix that prevented ``taskgraph``
from avoiding recomputation of already completed tasks.