[server][client] Replace the word "Issue" with "Incident"

TODO:
We shoud add DB migration code because table names are
also renamed.

issues -> incidents
issueTrackers -> incidentTrackers
[DBAgentMySQL][DBAgentSQLite3] Add renametable()
[DBClientConfig][DBClientHatohol] Add DB migration code to rename issue related tables

issue_trackers-> incident_trackers
issues -> incidents
Merge pull request #325 from project-hatohol/issue-to-incident

Replace the word "issue" with "incident"