added a basic frameword to get 'functions' data
form a ZABBIX server.

Data of 'functions' is also obtained when processing
to get 'triggers' is performed as a side effect.

So we will add code to cache data of 'functions' and
 return it when getFunctions() is called.

<< Tasks to do in the near future >>
- implemented funtions to get the follwoing.
  + functions
  + items
  + hosts
- parse arguments of --data-store-zabbix and create multiple
  DataStoreZabbix instances.