Sphinx-needs

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* Improvement: New option **tags** for :ref:`needimport` directive
* Bugfix: Handling of relative paths in needs builder

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* New feature: Directive :ref:`needimport` implemented
* Improvement: needs-builder stores needs.json for all cases in the build directory (like _build/needs/needs.json) (See `issue comment <https://github.com/useblocks/sphinx-needs/issues/9#issuecomment-325010790>`_)
* Bugfix: Wrong version in needs.json, if an existing needs.json got imported
* Bugfix: Wrong need amount in initial needs.json fixed

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* Bugfix: Setting correct working directory during conf.py import
* Bugfix: Better config handling, if Sphinx builds gets called multiple times during one single python process. (Configs from prio sphinx builds may still be active.)
* Bugifx: Some clean ups for using Sphinx >= 1.6

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* Bugfix: Added missing dependency to setup.py: Sphinx>=1.6

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* Improvement: Builder :ref:`needs_builder` added, which exports all needs to a json file.

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* Bugfix: Build has crashed, if sphinx-needs was loaded but not a single need was defined.