Added separate context pointers for each of the global callbacks [#399]

Compilation fix: using enum SUCCESS instead of MQTTASYNC_SUCCESS

Installation of new headers

Fix missing headers included by MQTTAsync.h

Added below headers to installation:
MQTTProperties.h MQTTReasonCodes.h MQTTSubscribeOpts.h
as they are needed during build process.
Signed-off-by: Krzysztof Bogucki <kristofbo@gmail.com>

This commit fixes compilation error:
MQTTAsync.h:1114:143: error: invalid conversion
from ‘int’ to ‘MQTTReasonCodes’ [-fpermissive]
Signed-off-by: Krzysztof Bogucki <kristofbo@gmail.com>

prevent segfault for unknown protocol in test app