Replace CDTLoggingLevels variable define with a declaration

Replace CDTLoggingLevels variable define in CDTLogging.h with a declaration, moving the definition
into CDTLogging.m

Modify test set up to use macros instead of setting the levels in the array itself.

Also add CDT prefix to START_CONTEXT and END_CONTEXT respectively
Merge pull request #96 from cloudant/42602_make_cdtlogging_global

Remove variable define from header