Allow additional Direwolf configuration via Dirus

[kd8drx]

A bundle of updates to the code, primarily focused on improving the ability to manage settings using the dirus.json file. Testing was done on my MacBook Pro running macOS 10.13.6 and a Raspberry Pi 3 with Raspbian Jesse.

I do hope these features prove useful to the rest of the amateur radio community. This is a super helpful tool for automating some of these backend processes that make the hobby fun!

Features Added:

• Added a separate direwolf json dict that allows you to
  • Set a Direwolf config file via the path key
  • Set the path to Direwolf, assuming it's not in your $PATH
• Added Direwolf argument for sample_rate, ensuring it is synced with the sample rate used in rtl_fm. In my testing, this is often required to ensure proper decoding of packets.
• Updated README.rst with build, configuration, and contributor sections based.

Other Minor Changes:

• Changed internal variables for Direwolf and rtl_sdr commands to better differentiate between them, and make the code read a little better (I hope?)

• Small changes in the Direwolf commands to deal with Python 2's fun handling of strings vs. unicode.