Add support for repeated idle callbacks.

Useful for animations or any repeated action that is supposed to use up all available CPU time that is not needed for event processing.
Make Task weakly shared to allow it to be passed between thread.
Fixed the declaration of Task.m_fiber - __gshared seems to imply "static" and thus is unsuitable.
Merge branch 'master' of github.com:rejectedsoftware/vibe.d
Improved HTML log display.
Allow repeated calls of Win32Timer.stop() and properly reset m_pending.
Number worker thread names.