SgConnection added arg "fieldValuesOnly" to clearCache()

SgConnections clearCache() function now includes an arg "fieldValuesOnly" which
when set to True, default, the cache for field values will only be cleared and
all weakrefs to currently alive Entity objects will be left untouched.
Added global config option DEFAULT_CONNECTION_CACHING

ShotgunORM.config.DEFAULT_CONNECTION_CACHING is the default value that
SgConnections look at for determining whether to enable/disable Entity caching.
Changing this value will only affect new SgConnection objects and not any
pre-existing ones.