Merge Master into Dev (#745)

* Fix spi flash error when compiling in Arduino IDE 1.6.6

* Bump minor version

* Fixed "invalid suffix on literal" warning

* Fix addressing bug in _doSign bitfield

* Make sure nodes not supporting signing informs GW

A node that does not support signing, still needs to inform the
gateway about this to make sure the gateway carries a valid signing
requirement table when a node Id that was requiering signing stops
doing this.
This fixes #286.

* Prevent SecureActuator from accepting ACKs as commands

As ACKs are not currently signed, allowing ACKs as commands
is a considerable security hole for this sketch. This is now resolved.

* Backport of bugfix (#259) in repeaters when forwarding signed messages

* Bumped version to 1.5.2

* Corrected doSign variable name (was _doSign)

* Bumped version to 1.5.3

* Add NULL termination to payloads

Messages addressed to "this" node, will have a null char
appended to the payload after message verification is
done, because some "getters" assume the message being a
string.

library.json

@@ -7,7 +7,7 @@
     "type": "git",
     "url": "https://github.com/mysensors/MySensors.git"
   },
-  "version": "2.1.0-beta",
+  "version": "2.1.0",
   "frameworks": "arduino",
   "platforms": [
     "atmelavr",