update ItemCoVisitation below: - as cache size 0 leads to error, set the default value to 100. - in visit(), use list::splice() to move list for constant time. - in getCoVisitation(), filter out the input item in results, and sort   the items by frequency descreasingly.
update ItemCoVisitation below:
- as cache size 0 leads to error, set the default value to 100.
- in visit(), use list::splice() to move list for constant time.
- in getCoVisitation(), filter out the input item in results, and sort
  the items by frequency descreasingly.

pombredanne/idmlib,pombredanne/idmlib,izenecloud/idmlib,izenecloud/idmlib,izenecloud/idmlib,izenecloud/idmlib,izenecloud/idmlib,pombredanne/idmlib,pombredanne/idmlib