Make hooks execute in the order they are defined

The hooks defined via the hook directives were kept in a hash keyed by
the hook routine's address. Since they were executed as returned by the
'value' hash function the order wasn't really specified.

This commit substitute a list for that hash and makes sure that the
hooks are executed in the order they're defined.
CheckJira: add directive CHECK_JIRA_DISABLE

This directive globally disables all CHECK_JIRA directives. It's useful,
for instance, when your JIRA server must be taken down for maintenance
and you don't want to reject Subversion commits in this period.
TODO: internationalize the SVN::Hooks messages.
TODO: Implement a "case insensitive" plugin
TODO: Increment SVN::Hooks::Notify to send emails in HTML
Change indentation on Changes file
v1.26

  [Changes]

  - Add directive CHECK_JIRA_DISABLE to SVN::Hooks::CheckJira to disable all
    CHECK_JIRA directives.

  - Make hooks execute in the order they are defined. Previously, the
    execution order was unspecified. (Change kindly contributed by
    DC. Thanks!)