PlatformIO layout

[mumrah]

This PR reorganizes the source files to make the repo compatible with PlatformIO. It also creates a Arduino sketch with the same name as the directory which is a requirement of the Arduino IDE.

With thing change I can:

• Open "bitx40.ino" with Arduino IDE and compile/upload the Raduino (just like today). No need to "copy the sketch into a folder of the same name"
• Open as a PlatformIO project and compile/upload to the Raduino

What this allows:

• Development in PlatformIO (many nice things about this compared to Arduino IDE)
• Eventual refactoring of large sketch into smaller private libraries
• Easier writing of unit tests and integration with TravisCI

At first, I tried pulling the Si5351 code into a project-local library (which is supported by PlatformIO), but couldn't get it working via the INO sketch. Something to do with #include and my lacking C skills.

Is this something people would be interested in?

[davehun]

great suggestion I was just about to do the same thing!