MCP3002 support

[2bndy5]

• added MCP3002 support via new module and corresponding examples to address #2

  docs improvements

  Since I had to test differential reads pretty thoroughly, I ended up

• making the docs a bit more concise concerning the differential channel mappings and the driver class' channel aliases
• added return value ranges to applicable attributes
• added some links to datasheets for wiring/pinout (yea I screwed it up on the first try)
• AnalogIn's API contains a warning about the ADCs' neglect of negative numbers for differential read results and hints behavior for voltages outside reference voltage bounds. Although, I feel the placement of this warning might serve better at the top of the API references, but its fine for now.

  code improvements

• MCP3002's SPI calls transact over 2 bytes while MCP3004/MCP3008 still transact over 3 bytes
• removed redundantly setting each byte in the MCP3xxx's internal output buffer and removed the local variable command
• removed the constants used for the differential read flag & the start bit as the start bit is now set upon instantiation while differential read flag is still set during every read() call.
• modified the handling of the received data in the input buffer to use 1 less computation and removed local variable result
• removed unused object instance attributes negative_pin and channel from AnalogIn class as well as the __getitem__() related to the channel attribute.

All these changes were tested on a MCP3002 and MCP3008 for verification with consistent results.

[brentru]

@2bndy5 Travis is passing, merging in and releasing! Thanks for the contribution, this should close https://github.com/adafruit/Adafruit_CircuitPython_MCP3xxx/issues/2