issue,posting: numbering per tracker or board.
issue,posting: Add numbering fields in Project.

Do not compute the lastest number of issue or posting on the fly. Instead
just use Project.lastIssueNumber or lastPostingNumber.

For lastIssueNumber and lastPostingNumber,
* Use primitive type 'long' in order to set them to zero by default
* Set their visibility from public to private in order to prevent to
  update them by mistake
Merge pull request #5 from npcode/feature/issue-number-per-tracker

issue,posting: numbering per tracker or board.