Bug Fixes

• Fix circular references in SSL implementation to reduce
  the need for GC.
  (by @fantix in 3070ec8)

• Fix a memory leak in call_later() and call_at().
  The leak occurred when a callback argument had a reference to
  the event loop.
  (by @1st1 in 1a5dbc2 for #239)

• Fix compilation warnings.
  (by @JelleZijlstra in d9a111b)

• Round (instead of flooring) delay in call_later().
  This ensures that the callback is never called slightly before
  the specified time.
  (by @fantix in 8f037a6 for #233)