Improves in ESP32NVS library (#196)

* Improved ESP32NVS default value support for read* functions

Fixed uninitialized NVS flash storage. Added example code.

* Add methods for String and remove unnecessary nvs_flash_init() in ESP32NVS class

+ other minor declaration fixes

* ESP32NVS function name renaming

* read* -> get*
* write* -> set*
* erase -> clear / remove

libraries/ESP32NVS/library.properties

@@ -4,6 +4,6 @@ author=Hristo Gochkov
 maintainer=Hristo Gochkov <hristo@espressif.com>
 sentence=Provides frendly access to ESP32's Non-Volatile Storage
 paragraph=
-category=Storage
+category=Data Storage
 url=
 architectures=esp32