Refactor to proper PlatformIO library structure

- Move source files to src/ and headers to include/ directories
- Remove Arduino IDE specific files (.travis.yml, keywords.txt, library.properties, CMakeLists.txt, travis/)
- Update library.json with export section pointing to include/
- Add platformio.ini for development and testing
- Update .gitignore with PlatformIO-specific entries
- Update LICENSE with fork attribution
- Update README.md with PlatformIO-focused installation instructions

Verified working with:
- ESP8266 examples (Basic, Advanced)
- ESP32 examples (Basic)

All builds successful ✅

include/strings_en.h

@@ -0,0 +1,15 @@
+/**
+ * Contents of this file have moved to 2 new locations
+ * wm_strings_nn.h
+ *  wm_consts_nn.h
+ */
+
+#warning "This file is deprecated"
+
+#ifndef _STRINGS_EN_H_
+#define _STRINGS_EN_H_
+
+// strings files must include a consts file!
+#include "wm_strings_en.h" // include constants, tokens, routes
+
+#endif