Issue #23: Fix version reference errors for hotfix

* Fixes #23.
* bin/git-hotfix
  * Changed IssueIdNotSpecifiedException to VersionSpecifiedException.
  * Changed VersionSpecifiedException to reflect version instead of issue identifier.
* git-hotfix (documentation): changed to reflect version and not issue identifier.
Merge pull request #26 from jlmorgan/bugfix/issue-23

Issue #23: Fix version reference errors for hotfix