Repo improvements: PlatformIO library setup, improved .gitignore and .gitattributes, GitHub license detection

[hansmbakker]

Folder restructuring for PlatformIO and Arduino IDE

This PR simplifies the folder structure so that PlatformIO and Arduino IDE can include it. It also adds the metadata files (library.json for PlatformIO and library.properties for Arduino IDE).

Arduino expects all source and header files in the same directory called src.

PlatformIO is an IDE for embedded development that makes it easy to build and maintain Arduino projects.

By creating a library.json definition, it is possible to create a library that is easy to use in PlatformIO.

Conditional code inclusion

To prevent Arduino and PlatformIO automatically picking up cpp files for unused interfaces (like SoftwareSerial for SWHSU), compile flags are introduced. This also prevents compiler errors where SoftwareSerial is not available like on ESP32.

Git metadata files

A generated .gitignore file is added based on the c++ and PlatformIO technologies using https://gitignore.io/.
.gitattributes is added to prevent issues with cross-platform line endings.

Metadata

The metadata files automatically take a dependency on Don's NDEF library, so that it is not needed to maintain a copy in this repo.

License detection

This PR also moves the license file so that GitHub can recognize it.

Code fixes

This PR includes fixes from #126, #129 and #133. For simplicity I reapplied the code-changes rather than cross-merging branches, but I want to attribute @FStefanni, @JEphron and @redstonee here.

[FStefanni]

Hi,

thank you for the work!
Let's hope it will be merged asap, and not going stale as the other pr's.

Regards

[Cincinnatu]

Hello. I will check soon whether this is effective and reasonable.