Added a AppVeyor configuration file that can be used to implement Windows CI support

Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

appveyor.yml

+version: 1.0.{build}
+image: Visual Studio 2015
+configuration: Debug
+install:
+- cmd: cinst openssl.light
+build_script:
+- cmd: >-
+    mkdir build.paho
+
+    cd build.paho
+
+    "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64
+
+    cmake -G "NMake Makefiles" -DPAHO_WITH_SSL=FALSE -DPAHO_BUILD_DOCUMENTATION=FALSE -DPAHO_BUILD_SAMPLES=TRUE -DCMAKE_BUILD_TYPE=Release -DCMAKE_VERBOSE_MAKEFILE=TRUE ..
+
+    nmake
+
+    cd ..
\ No newline at end of file