Nsot

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* Implement GET/PUT objects by natural_key and minor fixes.

* General

+ Upgraded ``drf-nested-routers==0.11.1``
+ Re-organized nsot.api.urls to improve readability
+ Implemented natural_key mappings for Device and Network resources

* Networks

+ Updated ``Network.objects.get_by_address()`` to support optional site=
argument for filtering by site_id.

* Serializers

+ Moved ``.create()``, ``.update()`` methods from Device, Network serializers
to new ``ResourceSerializer`` base.

* Change Events

+ Fix when deleting a resource object using the API failed for any
reason the "Delete" change event would still be created. The Change
event will now only be kept *after* a successful delete.

* Views

+ Implemented ``NsotViewSet.get_object()`` support for nested serializers
+ Updated Network lookup_value_regex to support loookup by pk or
IPv4/IPv6 natural_key.

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* Fix 142 - Properly catch non-serializer errors in API views.
* This includes unique constraints and integrity errors.
* Added a regression test for this error case.

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* Fixes 82: Implemented for regex-based attribute lookups via the API.
* You now may append ``_regex`` to an attribute name in a set query and
provide a regex pattern as the value to perform regex-based lookups.

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* Implement API filtering for value objects & perf. tweaks (Fixes 137)
* Value objects now have a site_id attribute that is hidden and
automatically populated by their parent Attribtue, similarly to
Interface objects and their parent Device.
* New API endpoint: ``/api/sites/:site_id/values/``
* Improved performance when creating/updating Interface objects by not
explicitly looking up the parent Device object EVERY time.

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* Small tweaks to web UI
* Site index page

+ Interface count now added to Site dashboard
+ Links for ipv4/6 and network usage now link to filtered Network list
+ "Changes" renamed to "Recent Changes"
+ Under "Network Usage", "In Use" renamed to "Assigned"

* Networks list

+ Added "ip_version" filter
+ Columns now link to filtered Network list

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* Upgrade to Django==1.8.7 and DRF==3.3.2
* Filter fields now implemented in Browsable API (new in DRF 3.3)
* Added django-crispy-forms as a dependency
* Bootstrap JS updated to v3.3.5
* Bower updated to include Bootstrap fonts (DRF 3.3. needs this)