[IncidentSenderRedmine] Revise a member function name

getIssuesURL ->
getIssueURL

Because it returns a URL for one issue.
[IncidentSenderRedmine] Remove project ID from post URL

Specify project ID by JSON instead.

Although I implicitly expected Rails's typical behavior, such
interface isn't described in the Redmine's document:

  http://www.redmine.org/projects/redmine/wiki/Rest_Issues
Merge pull request #349 from project-hatohol/fix-redmine-post-url

Fix post URL for Redmine's issues