implemented VirtualDataStoreZabbix::getTriggerList().

Unfortunately, this patch makes the following problem.
  testFaceRest::test_triggers:                          [INFO] <FaceRest.cc:51> started face-rest, port: 53194
[INFO] <FaceRest.cc:150> Got Exception: <DBAgentSQLite3.cc:250> Failed to call sqlite3_prepare(): 1
/home/kyamato/work/asura/src/.libs/libasura.so.0, AsuraException::saveStackTrace() +0x1a, _ZN14AsuraException14saveStackTraceEv+0x1a [0x7f9d318897da]
/home/kyamato/work/asura/src/.libs/libasura.so.0, AsuraException::AsuraException(std::string const&, char const*, int) +0xa0, _ZN14AsuraExceptionC1ERKSsPKci+0xa0 [0x7f9d318898f0]
/home/kyamato/work/asura/src/.libs/libasura.so.0, DBAgentSQLite3::getTriggerInfoList(std::list<TriggerInfo, std::allocator<TriggerInfo> >&) +0x40e, _ZN14DBAgentSQLite318getTriggerInfoListERSt4listI11TriggerInfoSaIS1_EE+0x40e [0x7f9d3188b7ae]
/home/kyamato/work/asura/src/.libs/libasura.so.0, VirtualDataStoreZabbix::getTriggerList(std::list<TriggerInfo, std::allocator<TriggerInfo> >&) +0x1b, _ZN22VirtualDataStoreZabbix14getTriggerListERSt4listI11TriggerInfoSaIS1_EE+0x1b [0x7f9d318c6e9b]
/home/kyamato/work/asura/src/.libs/libasura.so.0, FaceRest::handlerGetTriggers(_SoupServer*, _SoupMessage*, char const*, _GHashTable*, SoupClientContext*, void*) +0x3b, _ZN8FaceRest18handlerGetTriggersEP11_SoupServerP12_SoupMessagePKcP11_GHashTableP17SoupClientContextPv+0x3b [0x7f9d318933bb]
/home/kyamato/work/asura/src/.libs/libasura.so.0, FaceRest::launchHandlerInTryBlock(_SoupServer*, _SoupMessage*, char const*, _GHashTable*, SoupClientContext*, void*) +0x16, _ZN8FaceRest23launchHandlerInTryBlockEP11_SoupServerP12_SoupMessagePKcP11_GHashTableP17SoupClientContextPv+0x16 [0x7f9d31892d76]
<snip>