[NimBLEServer] Support duplicate characteristics (#36)

* Add method to get all characteristics of a service

* Added method on NimBLEDescriptor to get the value out as a std::string

* Added methods to get things by handle where UUID could be used already.

* Added helper methods for getting a list of characteristics with a given UUID and service

* Demote the warning log for adding duplicate characteristics to a debug level log.

* Add methods to get services and characteristics using UUID + index

src/NimBLEDescriptor.h

@@ -44,20 +44,23 @@ class NimBLEDescriptorCallbacks;
 class NimBLEDescriptor {
 public:
     uint16_t     getHandle();
-    size_t       getLength();
     NimBLEUUID   getUUID();
-    uint8_t*     getValue();
-    void         setCallbacks(NimBLEDescriptorCallbacks* pCallbacks);
-    void         setValue(const uint8_t* data, size_t size);
-    void         setValue(const std::string &value);
     std::string  toString();
 
+    void         setCallbacks(NimBLEDescriptorCallbacks* pCallbacks);
+
+    size_t       getLength();
+    uint8_t*     getValue();
+    std::string  getStringValue();
+
+    void         setValue(const uint8_t* data, size_t size);
+    void         setValue(const std::string &value);
     /**
      * @brief Convenience template to set the descriptor value to <type\>val.
      * @param [in] s The value to set.
      */
     template<typename T>
-    void setValue(const T &s) {
+    void         setValue(const T &s) {
         setValue((uint8_t*)&s, sizeof(T));
     }