Fixes per sommersoft's review

[brentru]

• Added explicit check for the wifi kwarg against adafruit_esp32spi_wifimanager.ESPSPI_WiFiManager to make sure it's it's an instance of _WiFiManager, not something else accidentally passed in
• Changed api.rst to reflect directory structure
• Changed _compose_path to a more readable path.

Changes requested per: #1

[sommersoft]

Looking over the Travis build, and it's Sphinx failure on line #388, autodoc is failing on the import of neopixel contained in adafruit_esp32spi, which is imported in adafruit_io.py.

Since requirements.txt pip installs adafruit_esp32spi on the Travis instance, Sphinx can locate that module in the sys.path. However, neopixel isn't available in the sys.path, since that CircuitPython module isn't pulled into the instance.

There are two options here:

1. add CircuitPython to the intersphinx mapping. Example
2. add an autodoc_mock_imports including neopixel to conf.py.

[brentru]

Travis is passing, merging into master