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/examples/StartCounter/StartCounter.ino

@@ -0,0 +1,56 @@
+/*
+ ESP32 start counter example with Non-volatile storage
+
+ A simple example which use Non-volatile storage on ESP32 to store how many
+ times ESP32 module was started.
+
+ created for arduino-esp32 09 Feb 2017
+ by Martin Sloup (Arcao)
+*/
+
+#include <ESP32NVS.h>
+
+NVSClass keyStorage;
+
+void setup() {
+  Serial.begin(115200);
+  Serial.println();
+
+  // Open NVS key storage with my-app namespace. Each application module (library, etc.)
+  // have to use namespace name to prevent key name colisions. We will open storage in
+  // RW-mode (second parameter has to be false).
+  // Note: Namespace name is limited to 15 chars
+  keyStorage.begin("my-app", false);
+
+  // Clear an opened key storage
+  //keyStorage.clear();
+
+  // Or remove the counter key from key storage
+  //keyStorage.remove("counter");
+
+  // Get a counter value, if key is not exist return default value 0
+  // Note: Key name is limited to 15 chars too
+  unsigned int counter = keyStorage.getUInt("counter", 0);
+
+  // Increase counter
+  counter++;
+
+  // Print counter to a Serial
+  Serial.printf("Current counter value: %u\n", counter);
+
+  // Store counter to the key storage
+  keyStorage.setUInt("counter", counter);
+
+  // Close the key storage
+  keyStorage.end();
+
+  Serial.println("Restarting in 10 seconds...");
+
+  // Wait 10 seconds
+  delay(10000);
+
+  // Restart ESP
+  ESP.restart();
+}
+
+void loop() {}