[WIP] Refactor and convert to Girder 3.

This separates out the various components to separate python packages:
  - large_image[sources,memcached,all]

The tile sources are each their own package.  Most of them have an
extra_requires of `[girder]` that also installs the girder tile source.
  - large_image_source_dummy
  - large_image_source_mapnik[girder]
  - large_image_source_openslide[girder]
  - large_image_source_pil[girder]
  - large_image_source_tiff[girder]
  - large_image_source_test

The Girder plugin (without annotations):
  - girder_large_image[tasks]

The annotation models and web client plugin:
  - girder_large_image_annotation

The worker task:
  - large_image_tasks[girder,worker]

numpy and PIL are now hard requirements.

There were some inconsistencies in Python 2 and Python 3 error
conditions (for instance, the exact exception raised when a negative
width thumbnail is requested) that have been made more consistent.

Before, you could get a Girder tile source from an item id via
`large_image.getTileSource('girder_item://<itemId>')`.  This has changed
to `girder_large_image.getGirderTileSource(<item or itemId>)`.

Still needed:
- All frontend tests
- Documentation

.editorconfig

@@ -0,0 +1,36 @@
+root = true
+
+[*]
+indent_style = space
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8
+
+[*.js]
+indent_size = 4
+
+[{*.json}]
+indent_size = 4
+
+[Makefile]
+indent_style = tab
+
+[*.pug]
+indent_size = 2
+
+[*.py]
+indent_size = 4
+max_line_length = 100
+
+[{*.rb,Vagrantfile}]
+indent_size = 2
+
+[{*.sh,*.bat}]
+indent_size = 4
+
+[*.styl]
+indent_size = 2
+
+[*.yml]
+indent_size = 2

.gitignore

@@ -1,10 +1,56 @@
+# Byte-compiled / optimized / DLL files
+__pycache__/
 *.py[cod]
-node_modules
-_build*
+*$py.class
+
+# C extensions
+*.so
+
+# Distribution / packaging
+.Python
+env/
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+parts/
+sdist/
+var/
+wheels/
+*.egg-info/
+.installed.cfg
+*.egg
+
+# Installer logs
+pip-log.txt
+pip-delete-this-directory.txt
+
+# Unit test / coverage reports
+htmlcov/
+.tox/
+.coverage
+.coverage.*
+.cache
+coverage.xml
+*.cover
+.hypothesis/
+.pytest_cache/
+
+# pyenv
+.python-version
+
+# dotenv
+.env
+
+# virtualenv
+.venv
+venv/
+ENV/
+
+node_modules/
+
 .*.swp
 .*.swo
-env
-dist
-build
-*.egg-info
-.pytest_cache
+*.aux.xml