Add new spinel property THREAD_NEIGHBOR_TABLE and its support in Ncp (#742)

This commit adds a new spinel property `THREAD_NEIGHBOR_TABLE` and
its get handler in `NcpBase` class.

doc/spinel-protocol-src/spinel-tech-thread.md

@@ -185,7 +185,7 @@ Specifies the self imposed random delay in seconds a REED waits before
 registering to become an Active Router.
 
 ### PROP 5386: PROP_THREAD_PREFERRED_ROUTER_ID
-* Type: Write
+* Type: Write-Only
 * Packed-Encoding: `C`
 
 Specifies the preferred Router Id. Upon becoming a router/leader the node
@@ -193,3 +193,19 @@ attempts to use this Router Id. If the preferred Router Id is not set or
 if it can not be used, a randomly generated router id is picked. This
 property can be set only when the device role is either detached or
 disabled.
+
+### PROP 5387: SPINEL_PROP_THREAD_NEIGHBOR_TABLE
+* Type: Read-Only
+* Packed-Encoding: `A(T(ESLCcCbLL))`
+
+Data per item is:
+
+* `E`: Extended/long address
+* `S`: RLOC16
+* `L`: Age
+* `C`: Link Quality In
+* `c`: Average RSS
+* `C`: Mode (bit-flags)
+* `b`: `true` if neighbor is a child, `false` otherwise.
+* `L`: Link Frame Counter
+* `L`: MLE Frame Counter