Simplify API documentation by using doctests (#73)

* Add doctests to SmartBulb

* Add SmartDevice doctests, cleanup README.md

* add doctests for smartplug and smartstrip

* add discover doctests

* Fix bulb mock

* add smartdimmer doctests

* add sphinx-generated docs, cleanup readme a bit

* remove sphinx-click as it does not work with asyncclick

* in preparation for rtd hooking, move doc deps to be separate from dev deps

* pytestmark needs to be applied separately for each and every file, this fixes the tests

* use pathlib for resolving relative paths

* Skip discovery doctest on python3.7

The code is just fine, but some reason the mocking behaves differently between 3.7 and 3.8.
The latter seems to accept a discrete object for asyncio.run where the former expects a coroutine..

docs/Makefile

@@ -0,0 +1,20 @@
+# Minimal makefile for Sphinx documentation
+#
+
+# You can set these variables from the command line, and also
+# from the environment for the first two.
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+SOURCEDIR     = source
+BUILDDIR      = build
+
+# Put it first so that "make" without argument is like "make help".
+help:
+	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+
+.PHONY: help Makefile
+
+# Catch-all target: route all unknown targets to Sphinx using the new
+# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
+%: Makefile
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)