MOBSDK-559/495

MOBSDK-559: use UTF8 strings instead of ASCII in both AlfrescoSDK & CMIS library
MOBSDK-495: respond to authentication challenges posed by self-certified servers. Both AlfrescoSDK and CMIS lib

As for MOBSDK-495: the CMIS library does a few more checks in case an authentication challenge is sent. We may want to consider implementing something similar (if relevant)?